Adding support for the Certificate managed object

This change polishes and reorganizes the implementation of the
Certificate managed object and its required subclasses. It adds in
documentation and test suites for all modified classes and updates the
SecretFactory to support creating default Certificate objects.

kmip/core/primitives.py

@@ -420,8 +420,7 @@ class Enumeration(Integer):
         return "{0}(value={1})".format(type(self).__name__, self.enum)
 
     def __str__(self):
-        return "{0} - {1} - {2}".format(
-            type(self.enum), self.enum.name, self.enum.value)
+        return "{0}.{1}".format(type(self.enum).__name__, self.enum.name)
 
 
 class Boolean(Base):