Primary Key Fixes

[sethdenner]

• Fixed ColumnFamilyModel._get_pk_val to handle ForeignKey valued properly.
• Fixed PrimaryKeyField get_prep_value to handle ForeignKeys properly.
• Fixed FieldUUID and AutoFieldUUID get_prep_value methods to behave properly convert UUID values (and avoid trying to convert them to int).
• Fixed DateTimeField get_prep_value to always use 3 most significant digits.
• Modified unit tests to catch these edge cases.
• Refactored PrimaryKeyValue to values module since both fields and models modules require it.
• Incremented patch version to 0.7.4

[coveralls]

Coverage increased (+0.3%) to 66.878% when pulling ddfd5b859d8902df978ed896ce8fca4a29253fbd on sethdenner:primary-key-fixes into cacf9a19815636ac90ca6b92fec403120a263515 on Knotis:master.