classes to eliminate issues with duplicated objects. closes#611, #612, #613, #618
* NSEntityDescription is now aware of the primaryKeyAttribute. Can be configured via
Interface Builder within Xcode or programatically.
* Added findByPrimaryKey: interface to the Core Data extensions.
* Relaxed dependencies on RKManagedObjectMapping across the system now that primaryKey is
available without a reference to the mapping.
* Reorganized tests to accommodate split into Logic & Application.
* Eliminated 'Spec' naming in favor of 'Test' as the suite is entirely based on SenTest.
* Pulled majority of testing support classes up into the library and documented them.
* Introduced RKApplicationTests app for running the RKTableController UI tests
