Gpkit

- New model for considering constraints: all constraints are considered as sets of constraints which may contain other constraints, and are asked for their substitutions / posynomial less than 1 representation as late as possible.
- Support for calling external code during an SP solve.
- New class KeyDict to allow referring to variables by name or with objects.
- Many many other bug fixes, speed ups, and refactors under the hood.

[Documentation at time of release](http://gpkit.readthedocs.org/en/v0.4.0/)

Includes numerous improvements:
- Modular / model composition fixes and improvements
- Working controlpanel() for Model (!!!)
- ipynb and numpy dependency fixes
- printing fixes
- El Capitan fix
- slider widgets now have units