integrated timer, added score and timer to game screen in the middle.
scoreboard debugged, centered and pretty. Instruction text moved to where I think it belongs, but not tested. Need to remove it from game screen once it's properly placed.
Fixed background draw
Merge commit 'e39e5f00dbc52d658349b9fa5feeebef98818e0a' into aaron_mac

Conflicts:
	src/GameScreen.cpp
Player starting positions & resetting from level files; object size change in level file