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 
each test sets the 'cors' configuration property based on the passed value https://github.com/recurly/recurly-js/blob/cors/test/unit/token.test.js#L28