Move Floor-Detection Support to Superclass

Adds an EnhancedSprite class to hold enhancements to the base FlxSprite
class that will be shared by multiple objects, such as the workaround
for HaxeFlixel's broken floor detection.
Add Sound for Monsters Hitting the Floor

Also makes Monster an EnhancedSprite and adds support for telling it if
it's on the ground.
Reduce Volume of Hero Jump
Add Better Monster Drop Sound

Version 3. Don't ask about version 2.