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 
`XDomainRequest`, according to MSDN, supports the `contentType` property; however, in practice, this is not the case, and broswers implementing XDR in fact do not send the Content-type header. This method is only available on XHR2, thus the check.