Allow cross-origin requests to the GoGreen REST API

The REST API in GoGreen cannot be accessed via AJAX because of the same-origin policy of browsers.

Two techniques exist to circumvent this: JSONP [1] and CORS [2]. We should add one of these techniques to GoGreen. Preferably CORS, since it is less hacky, more secure (the client can still parse the data itself instead of automatically doing an 'eval' of the response) and better configurable.

[1] http://en.wikipedia.org/wiki/JSONP
[2] https://developer.mozilla.org/en-US/docs/HTTP/Access_control_CORS