An automatic build script and local settings

Requires #37, closes #20.

The script checks for each of the supported solvers, and chooses a default
solver from those that work.

The default solver (and all other settings, but it's the only one right
now) can be accessed via:

from gpkit import settings
settings['defaultsolver']
I can't believe I wrote a new diff tool. I can't believe Windows didn't have one on the command line.