[FEATURE] Changes towards enabling search to be extended by Media bundle

This PR:

- allows SuluSearch metadata objects to be modified by event listeners.
- prepends a configuration to MassiveSearchBundle to use a custom Factory object so that we can create our own (Sulu) classes.

This is required to enable bundles such as MediaBUndle to remain uncoupled from SearchBundle.

Depends: 

- https://github.com/sulu-cmf/sulu/pull/260
- https://github.com/massiveart/MassiveSearchBundle/pull/10

## todo

- [x] Reviewed by Dan
- [ ] Reviewed by others

The metadata is the ****StructureCache. This just maps that to an IndexMetadata object.