Replace XML-RPC with REST API

Issue Summary

Replace the low-level XML-RPC communication with a new REST API.

With that being said, here's the plan:

1. Replace the XML-RPC communication and response handling with REST/drupal_http_request(), but still keep everything in mollom().
2. Ensure the REST API understands the identical request parameters and returns the identical values.
3. Ensure that error handling still works as expected.
4. For now, Drupal 7 resp. PHP5 only.

   In Drupal 6, we can't rely on native PHP5 XML helpers and Drupal's built-in XML parser helper functions (relying on xml_parse()) is not able to parse the returned XML for unknown reasons currently. Therefore, separate issue.

5. To focus on the actual replacement, the current patch simply converts/maps the old mollom.fooBar methods to new REST methods within mollom().

   This may change once the functionality works - OTOH, each method can only be called in a unique way, so instead of adding separate $request_method and $path parameters to mollom() and updating all the calling code, it might make more sense and be smarter to keep this idea/direction and advance on it instead (PHP class).

6. Especially the new CAPTCHA methods need attention.

   The old XML-RPC API always had to be called in order to retrieve a CAPTCHA URL, whereas the new REST way simply allows to retrieve a CAPTCHA in different formats on a certain URL. This requires us to parse out, store/remember, and re-use the server name and session ID. The consequence of doing so is that the client-side server/load balancing only kicks in once, initially when requesting/creating the CAPTCHA resource.

7. Various methods are still missing in the new REST API on the backend. Therefore, not everything can be converted yet.