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. 
right now it looks like these will fail if the params structure doesn't have all of the fields?  is that the desired behavior?  does it make sense for any of them to have defaults?