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 
`gravy` needs to scope to the window so that it's available globally. Using `var` would scope it to this module.

invoked here: https://github.com/recurly/recurly-js/blob/master/test/server/index.jade#L16