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 guess this is okay since the option defaults to false. I guess I'm just expecting it'll get copy/pasted to a Boolean option that defaults to true and cause problems.