Add plugin support for the facet system

Adds:
- World plugin support for the facet system. You can now add FacetProviders and Rasterizers from external modules into the core world.
- Configuration for FacetProviders
- A nicer base world generator with default facet system features

Behavior:
- A world generator needs to opt in to use plugins
- A world generator has control of where plugins are inserted
- Features can be completely turned off by setting key config options to zero

Other notes:
- It is feels a little strange using the WorldConfigurator because it is in essence just a Map<String, Component>.  Is there something to be added by wrapping it in this manner that I am not seeing?
- Do the default ranges for Core's providers look ok?

Screenshots
http://forum.terasology.org/threads/facet-provider-configuration.1091/