* 'uri': the URI for the request (required)
* 'query': an array of key-value pairs sent in a GET request
* 'post': an array of key-value pairs sent in a POST request
* 'callback': to be called after the request is finished. Should accept
a Phergie_Plugin_Http_Response object and return either the shortened
url or false if an error has occured.
If the 'post' key is present a POST request shall be made; otherwise
a GET request will be made. The 'post' key can be an empty array and
a post request will still be made.
If no callback is provided the contents of the response will be returned.