RobustModel should inherit from Model

[whoburg]

Carry over from #3, seems to me that RobustModel ought to inherit from gpkit.Model.

[1ozturkbe]

I know... but not even the basic building blocks of RobustModel inherit from nomials, so this is going to take a lot of work. It is on my mind, believe me.

[whoburg]

Great! Just figure having an issue is a good way to track it over time.