Implemented Golems, EnderDragon, EnderCrystal.
Fixed plugin command registration.

* Fixed plugin container lookups.
* Fixed getPlugin(id).
* Fixed wrong super in MixinEntitySnowman.
Add basic support for PlayerJoin/PlayerQuit events.

The underlying events are fired at a bad time for messing with the messages,
so that is not supported yet.
Implement basic MessageBuilder
Make the license plugin consistent with SpongeAPI
Implement Title API
Update mixin submodule