-Ball.changeMass() now changes the mass by an amount given by the
   'change' parameter.
-The level class now controls the amount of mass changed when the
   buttons 'u' or 'i' are pressed.
Merge branch 'master' of github.com:pczupil/TooAmbitious

Conflicts:
	BungeeBall/core/src/com/mygdx/bungeeball/Ball.java
	BungeeBall/core/src/com/mygdx/bungeeball/Level.java