Allow factory functions for creating API modules.

Allow factory functions for creating API modules.

This enables factory functions to be registered for API modules,
in addition to the module class itself. This allows modules to
use proper dependency injection via the modules constructor.

Example:

  $wgAPIModules['foo'] = array(
    'class' => 'ApiFoo',
    'factory' => function( $main, $action ) { ... }
  )

Change-Id: Ieb85493a7765f466317f5fa74b0b0e262220deab