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)

doxygen doc please.  especially for the gravity_visual_magnitude param