[20160912] Primitive Reaction Force based Control

WBOSC originally control CoM by proposing contact constraints and make CoM task. However, Gazebo simulator does not like contact constraint, so I decided to control reaction force directly as a task rather than relying on contact constraints.

I changed

• r5_urdf_rbdl.urdf: to have 8 virtual contact points
• controller_Balance: add reaction force control
• task_library: add reaction force control test task

The simulator shows a little better balance capability although it eventually diverge. At least, it does not flip the feet.