Snap footstep marker to planes

snap footstep marker to the planes automatically only if ~use_plane_snap is set to true

before snapping:
![screenshot_from_2014-06-14 22 30 29](https://cloud.githubusercontent.com/assets/40454/3278591/9dbab50a-f3c8-11e3-9ba4-7ca33cc7c2aa.png)

after snapping:
![screenshot_from_2014-06-14 22 30 41](https://cloud.githubusercontent.com/assets/40454/3278592/a13c1192-f3c8-11e3-8e3e-a9c5b31db42c.png)