Reorganizing message payload code

This change is primarily a reorganization of the message payload code
originally found in kmip/core/messages/operations.py. The code from that
module has been moved to a new package, kmip.core.messages.payloads, and
split into separate modules according to each supported KMIP operation.

The surrounding KMIP codebase has been updated to use the new package
structure and a placeholder test suite has been put in place for future
tests. This change does NOT include test cases as these would primarily
be a refactoring from the current test suite found in
kmip/tests/core/messages/test_messages.py and that work has not been
done yet.

kmip/core/messages/payloads/create.py

@@ -0,0 +1,113 @@
+# Copyright (c) 2014 The Johns Hopkins University/Applied Physics Laboratory
+# All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
+from kmip.core import attributes
+from kmip.core import enums
+from kmip.core.enums import Tags
+
+from kmip.core.objects import TemplateAttribute
+
+from kmip.core.primitives import Struct
+
+from kmip.core.utils import BytearrayStream
+
+
+class CreateRequestPayload(Struct):
+
+    def __init__(self,
+                 object_type=None,
+                 template_attribute=None):
+        super(self.__class__, self).__init__(tag=enums.Tags.REQUEST_PAYLOAD)
+        self.object_type = object_type
+        self.template_attribute = template_attribute
+        self.validate()
+
+    def read(self, istream):
+        super(self.__class__, self).read(istream)
+        tstream = BytearrayStream(istream.read(self.length))
+
+        self.object_type = attributes.ObjectType()
+        self.template_attribute = TemplateAttribute()
+
+        self.object_type.read(tstream)
+        self.template_attribute.read(tstream)
+
+        self.is_oversized(tstream)
+        self.validate()
+
+    def write(self, ostream):
+        tstream = BytearrayStream()
+
+        # Write the object type and template attribute of the request payload
+        self.object_type.write(tstream)
+        self.template_attribute.write(tstream)
+
+        # Write the length and value of the request payload
+        self.length = tstream.length()
+        super(self.__class__, self).write(ostream)
+        ostream.write(tstream.buffer)
+
+    def validate(self):
+        # TODO (peter-hamilton) Finish implementation.
+        pass
+
+
+class CreateResponsePayload(Struct):
+
+    def __init__(self,
+                 object_type=None,
+                 unique_identifier=None,
+                 template_attribute=None):
+        super(self.__class__, self).__init__(tag=enums.Tags.RESPONSE_PAYLOAD)
+        self.object_type = object_type
+        self.unique_identifier = unique_identifier
+        self.template_attribute = template_attribute
+        self.validate()
+
+    def read(self, istream):
+        super(self.__class__, self).read(istream)
+        tstream = BytearrayStream(istream.read(self.length))
+
+        self.object_type = attributes.ObjectType()
+        self.unique_identifier = attributes.UniqueIdentifier()
+
+        self.object_type.read(tstream)
+        self.unique_identifier.read(tstream)
+
+        if self.is_tag_next(Tags.TEMPLATE_ATTRIBUTE, tstream):
+            self.template_attribute = TemplateAttribute()
+            self.template_attribute.read(tstream)
+
+        self.is_oversized(tstream)
+        self.validate()
+
+    def write(self, ostream):
+        tstream = BytearrayStream()
+
+        # Write the contents of the request payload
+        self.object_type.write(tstream)
+        self.unique_identifier.write(tstream)
+
+        if self.template_attribute is not None:
+            self.template_attribute.write(tstream)
+
+        # Write the length and value of the request payload
+        self.length = tstream.length()
+        super(self.__class__, self).write(ostream)
+        ostream.write(tstream.buffer)
+
+    def validate(self):
+        # TODO (peter-hamilton) Finish implementation.
+        pass