Added maxElapsed variable.

When using variable timestep, "maxElapsed" will prevent "elapsed" from being too high, which can cause very jerky movement and erratic behavior, especially when the game lags for a second. There is corresponding change in FlxGame.
Changed maxElapsed default to 0.1

Also corrected "prevet" typo in comment
Merge pull request #965 from Vascofr/patch-2

Added maxElapsed variable.