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)

I've changed the variables to be called xmin/max. However we can't use Inf because it is mapped to a slider. You are correct, 100 is arbitrary, but the user can specify other limit through minrange and maxrange if desired. I also thought of using velocity limits but not all URDFs specify it. Any other ways to compute a good velocity range?