Moves /area/entered to its own file.
Adds a new area var called has_gravity. Determines if floor tiles count for movement control (planning on adding more, currently can only be badmined)
Adds a new mob var called lastarea that is updated with the area you're in every time /area/entered is called.
git-svn-id: http://tgstation13.googlecode.com/svn/trunk@2917 316c924e-a436-60f5-8080-3fe189b3f50e