Added gun and made a collision filter that rejects all collisions
Fixed "jump" bug, added gun image and logic, and added test single-player version of the tank game

From what I can tell, there were two causes of the jump/lag bug previously described.  Without my
knowledge the update(60) call in LWJGLRenderer actually blocked the thread for about 16 milliseconds
to even out the frame rate - in reality, my Thread.sleep() call was never actually executed.  While
remaining, for the most part, steady, the update(60) call occasionaly returned after only 4 milliseconds,
causing a jump as the next update call was executed sooner.  This was fixed by not specifying any
framerate.  This causes it to default to 60fps, without the occasional short finish bug, however.  Another
cause in multiplayer was that since the server sent snapshots at 60 times a second and the client
rendered at 60 frames a second, thread scheduling issues could sometimes result in the server sending
two update messages inbetween rendering calls, causing the tank to jump.  The fix to this is rendering
at a much higher rate than the server is transmitting, or using interpolation.
Merge branch 'master' of https://github.com/spfrommer/phage2d