Merge pull request #520 from bvssvni/master

Ignore Cargo.lock
intersperse updates and frames more precisely
add GameWindow::close()
Merge pull request #525 from bfops/close-window

add GameWindow::close()
in game_iterator, sleep until next frame *OR UPDATE*
change tail recursion to explicit loop
remove PrepareUpdateLoopState; change asserts to if
more comments from https://github.com/PistonDevelopers/piston/pull/524
remove fail in game iterator loop
Merge pull request #524 from bfops/master

intersperse updates and frames more precisely
Simplified condition in update loop

Closes https://github.com/PistonDevelopers/piston/issues/526