Sunk more of the comparison implementation into FBTestSnapshotController.

The original intent of https://github.com/facebook/ios-snapshot-test-case/pull/8 was to enable other types of test classes to use snapshots (to implement https://github.com/dblock/ios-snapshot-test-case-expecta). This meant allowing to set the path where snapshots are stored and the record mode. I realize that was pretty hacky, since the recorder still remained tightly coupled to the snapshot controller.

This is, I think, a better alternative. I sunk the methods in `FBSnapshotTestCase` into `FBTestSnapshotController`. The macros just use the controller to do the work. 

cc: @dstnbrkr