Merge pull request #50 from kul/master

unqote path in texture-atlas macro.
Remove radius arg from circle-shape
Throw exception for breaking change
Make update-fn! take variadic args
Add time travel
Don't save duplicate entities vectors in timeline
Fix exception text
Remove run! function
Rename draw-entity! to draw! to reduce redundancy
Make cell! private
Revert "Make cell! private"

This reverts commit b58b2155bbf435285bf5997392931cd918a5fb64.