Add Response class

- change Pod#pods to Pod#subpods for clarity
- add XML fixtures to test/fixtures
- define a FIXTURES constant (fixtures path) in the test helper
- require pretty print in the test helper for quick debugging
Remove ruby 1.9.3 from build matrix

Ruby 1.9.3 doesn’t have keyword arguments, which are super nice, and
anyway, Mac OSX ships with at least ruby 2.0.0. So....