Adding server support for the GetAttributes operation

This change adds server support for the GetAttributes operation.
The user can specify an object ID and an attribute list to get
the values of the attributes listed from the specified object.
The user can also omit either argument; the server will default
to using the ID placeholder and all viable attributes respectively.
Only a subset of the standard attributes are supported right now.
New tests have been added to cover the new feature.

Closes #185
Closes #211
Closes #218

kmip/core/primitives.py

@@ -825,11 +825,10 @@ class TextString(Base):
 
     def __validate(self):
         if self.value is not None:
-            data_type = type(self.value)
-            if data_type is not str:
+            if not isinstance(self.value, six.string_types):
                 msg = ErrorStrings.BAD_EXP_RECV
                 raise TypeError(msg.format('TextString', 'value', str,
-                                           data_type))
+                                           type(self.value)))
 
     def __repr__(self):
         return "{0}(value={1})".format(type(self).__name__, repr(self.value))