Requests for comment/Configuration interface

As part of the configuration overhaul, we want to move configuration out of the global scope, and instead have it be obtained via Config objects.

Interface
This is the interface that is currently in MediaWiki core, as of version 1.23

Implementation
We have a (yes, to get rid of globals we must add one), which allows for core and extensions to register their config objects. The 'main' config object is used by MediaWiki core. Currently  initializes and provides core's Config object, but it should be moved to WebStart.php so it is available when initialization happens.