Adds CORS support

##### Summary

- Adds a configuration parameter to shift all requests to use JSON+CORS via xhr instead of jsonp
- Removes caching scaffold
- Updates fixture server to handle POST requests
- Adds tests for plan, tax, & token

cc @bhelx @austinheap @cbarton @drewish 
the tokenSuite encapsulates expected `recurly.token` behavior regardless of calling signature. Thus above we run the suite twice: once with a plain object, and again with an HTMLFormElement. Each test is wrapped in the builder function, which constructs the first argument from the example.