Decoupled MocktailResponse from Mocktail.

Some Demeter action going on here.
Extract remainder of meaningful behavior to MocktailResponse.

MocktailResponse is now completely decoupled from MocktailURLProtocol
and Mocktail.  Mocktail serves as a loader and registry for MockTail
responses and MocktailURLProtocol serves as the delivery mechanism.

MocktailResponse can now be extracted for other interesting
applications. ;)
Exposed MocktailFileExtension