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 wraps any test suite which will hit the API fixture server. It simply calls the suite twice, leaving any logic of how to vary the run to the suite implementation.