Bring footstep planning into Drake

This pull request brings the footstep and walking planning systems currently used by DRC into Drake. It introduces a new interface class, LeggedRobot, and its subclass Biped, which owns the current walking planners. Use of the new tools is demonstrated by the runAtlasFootstepPlanning.m and runAtlasWalkingPlanning.m scripts in examples/Atlas. 

Planning on non-flat terrain is not yet handled here (that's all still inside DRC for the moment). 

Also, we're still planning to move the ZMP formulation out of examples/ZMP, but I'll save that for another pull request. 
Document class overview (and some methods)?