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 
I would like to add an additional validation here that the `data-recurly="token"` field is populated with a token; however, this builder function is not concerned with the outcome of the test, and the test is not concerned with the argument built for it.

I will think on this some, but believe an inelegant solution in the success test below will be the best option.