Updated readme, notebook, and gitignore
Updated readme, notebook to new GP class interface
new Geometric Program object, with constants sub
Merge pull request #17 from bqpd/gp-object

new Geometric Program object, with constants substitution
Merge branch 'master' into mosek-expopt

Conflicts:
	gpkit/minimize.py
	test_driven_development/simple_gp_example.py