Adding VCROrderedMutableDictionary, sorting -[VCRCassette JSON].

This approach adds all the data to an un-ordered instance of
VCROrderedMutableDictionary during JSON generation, but before it
returns, it iterates through the sorted keys in the dictionary and
returns a sorted version of the un-ordered instance as well.
Adding passing test covering JSON key ordering.
Adding NSString descriptions for VCRRecording and VCRRequestKey.
Moving VCROrderedMutableDictionary to where it belongs.

*shakes fist at Xcode's project folder management*
Merge pull request #2 from kreeger/ordered-json-hash

Adding VCROrderedMutableDictionary, sorting -[VCRCassette JSON].