implemented ellipsoid scaling with equivalent masses.  resolves #603

resolves the unhelpful scaling of the standard inertial ellipsoids.
draws each inertial ellipse as if the link was an ellipsoid of a known constant density.  now you can actually see the inertial effects of each joint, e.g. on Atlas:

![atlas_inertia](https://cloud.githubusercontent.com/assets/6442292/5370991/0cafbf54-7ffc-11e4-9bf6-5b655163a86b.png)