add draw_pause
add RewindElapsedTime
add pause handling to main
Merge branch 'fix/26/pause'

This adds pause handling to main. But it doesn't really enable it
as to make it work we need to make proper persistence mechanism.

Well, there are two ways to solve pause problem. One is just to
pause all input handling and freeze the world. The other is to
save the world's state and restore in case of resume or drop in
case of exit.

Yep, freezing is more efficient but uh.. not like I'm rendering
anything heavy here. How expensive it could be to remember all units
and later restore them on resume?

fix #26