Improved Entity interface.

* Removed getPosition, getX, getY, getZ in favor of getLocation
* Removed getVectorRotation in favor of getRotation
* Removed EulerDirection in favor of Vector3f
* Removed interact methods as they are meant for Players
* Removed eject in favor of setPassenger
* Removed dismount in favor of setVehicle
* Renamed getRiding in favor of getVehicle
* Renamed getRidden in favor of getPassenger
* Renamed teleport to setLocation
* Renamed isDead to isRemoved
* Updated javadocs
Remove flags from setLocation, add getBaseVehicle
Rename isValid -> isLoaded
Merge pull request #290 from SpongePowered/feature/entities

Improved Entity interface.