Improved Entity interface.

This PR is my first pass at improving the Entity interface. The following has been done -

* 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
* Renamed teleport to setLocation
* Renamed mount to setRidden
* Renamed eject to setRider
* Renamed getRiding to getRidden
* Renamed isDead to isRemoved
* Updated javadocs
Please specify the behaviour for rider and ridden to avoid inconsistencies between implementations.
This is also important for us users.