Adds CORS support

- Removes caching scaffold
- Adds json support to test server
- Adds plan, tax, and token tests
Merge pull request #157 from recurly/cors

Adds CORS support
Adds caching layer to Recurly#request

- Stores responses in an instance-bound hash
- Key is url+payload

Signed-off-by: Christopher Rogers <chrissrogers@gmail.com>