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 
This module returns either an `XHR` or an `XDomainRequest`, depending on presence of the `withCredentials`. This provides an effective fallback for when XHR does not support cross-domain requests (IE9)