Instead of relying on reference to the RKMapping, which could be dynamic, traverse from the attribute mapping back to its parent to obtain the appropraite date formatter.
* RKObjectParameterization now returns an empty dictionary if mapping produces an unmappable representation error (no mappable attributes/relationships found)
* Log errors if request construction results in an NSError and return nil
* Ensure errors are passed back up through object parameterization instead of just returning nil
* Reset the serialization MIME Type registry during RKTestFactory set up to ensure it is in a sane state
* Convert Hamcrest matchers to Expecta in the object mapper tests to fix infinite recursion on test failure (will need to completely phase out Hamcrest soon)
