Merge pull request #55 from dartsim/gazebo_support

Gazebo support
Merge pull request #56 from dartsim/gazebo_support

change the interface of World class.
fixed a bug in World::setState; modified cube to use world class
rewrote balance app
moved isSimulating flag to SimWindow
Merge remote-tracking branch 'refs/remotes/origin/master' into gazebo_support