Implement new api.php and load.php entry point

api.php:
- This interface allows loading of messages client-side, and
  exposes the same domains as the server-side interface.
  Transport format is JSON-only, including JSON-P support for
  cross-domain loading.

load.php:
- To make using this interface easier a small library was written.
- The interface requires jQuery to be loaded already.
- There is a custom adapter for MediaWiki environments (e.g.
  Tool Labs tools using MediaWiki and gadgets on Wikimedia wikis
  and elsewhere), which makes use of the mw.messages registry
  and the mw.message parser (and, if you load it, the jqueryMsg
  parser for magic words etc.)
- The standalone interface performs no parsing, so use it only
  if you know the messages of your domain don't need it.

Fixes #3.