Added JUnit tests for Entity and MovableEntity.
Added javadoc for Entity and MovableEntity. Also deceleration if now called friction.

ReverseSpeed has also been removed since it wasn't used.
AbstractProjectile, Abs.Tank, DefaultTank no longer require a reverseSpeed.
Fixed a bug where the turret would add its rotation instead of updating it. Also fixed so that the turret also follows the mouse while the mouse is clicked.