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 reason the request method checks are performed far from one another is that headers must be set on an xhr in the open state only, and the url must be known before the xhr is opened.