Refactored parts of FBSnapshotTestCase into FBSnapshotTestRecorder.

Related to https://github.com/facebook/ios-snapshot-test-case/issues/7, this just pulls out private functionality of `FBSnapshotTestCase` into `FBSnapshotTestRecorder`. Note that we now have to pass in `recordMode` and `selector` into this at test runtime.

With this change we can write a Specta test. See https://github.com/dblock/ios-snapshot-test-case-expecta for the implementation.

PS: I have signed the FB CLA.