Force visualizer

The forces are drawn in green, torques in purple (right hand rule) and gravity is in red for scale. You can visualize the wrenches by passing any velocity state(s) in the state_dims argument to inspector. So it assumes that if you want to visualize velocities, really you want to visualize forces and torques. It also classifies the forces by their type.

All of this is open to discussion obviously.


![complex_bird](https://cloud.githubusercontent.com/assets/2207326/4222239/8ed99874-3911-11e4-93bd-1868c07317c2.png)

all of this logic should go as a new method in RigidBodyForceElement... not here.  
not only will that keep it cleaner and more extensible, we'll be able to visualize forces during simulation, etc, too.