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. 
Not necessary for the initial commit, but it will be beautiful to have this derive from the constraint classes that are coming online, and have your optimization be able to use the entire constraint (and optimization) hierarchy.