Mocktail refactor

- Add support for simulated network latency
- Add support for a very basic placeholder system
- Move away from static methods for everything. The problem is that we now have quite some state. It was also extremely confusing that Mocktail was inheriting from NSURLProtocol.
- Split classes into dedicated files
- This is still completely API compatible with the old Mocktail. However, `startWithContentsOfDirectoryAtURL:` is now deprecated.