added spring actor

Created a spring actor that launches a character according to their
velocity before they hit the spring, however it has a limit to avoid
unintentional usage.

Also re-implemented the LadderActor with a more proper method (instead
of changing movement modes, now uses the StopMovementImmediately
function), and extended the map.
Merge pull request #3 from PurdueSIGGD/Initial-Springs

added spring actor