Refine footstep to snap plane

Refine footstep to ignore error of environment recognition

* before ![screenshot_from_2014-10-20 04 10 56](https://cloud.githubusercontent.com/assets/40454/4694320/ff8334d8-57c3-11e4-90c5-a182a54f2ae6.png)
* after ![screenshot_from_2014-10-20 04 11 11](https://cloud.githubusercontent.com/assets/40454/4694321/0096d8f2-57c4-11e4-9aa9-1ebc5a9c1e67.png)