Fixed issues with collision detection and resolution for slopes and one way plaftorms.

Specifically, players no longer "hover up" through one way platfroms, and they
can also no longer walk through floating or partial slopes. Also, slope peaks
(where a left slope and a right slope meet) are handled properly now.

With this change, jumping down from one way platforms now works correctly.
Merge branch 'master' of http://github.com/torinmr/platformer
Added comment.