Added collision detection and resolution
Added the intital physics code and some tests
Fixed the position error in render()
Added fixed timestepping
Properly integrated physics & some misc shuffling

Added gravity, scene etc.
Moved Vec2 to common.h
Adjusted main.cpp for new functionality
Simple cosmetic changes to test file
Added friction impulse code

physics code is in working state after this commit
Fixed the -INF bug of body velocity
Fixed the NAN impulse bug and some changes to test

Added example event handler
test build
Separated event and state handling

* cleaned up physics code for merge