Make gravmag.interactive.Moulder a stand-alone project

[leouieda]

This class is an interactive matplotlib plot for grav 2D modeling using polygons. It should be its own project and depend on Fatiando. Then we can make changes to it without being tied to the slow Fatiando release cycle. It would also be good to invoke it from the command-line and have a GUI way to save and load models/data.

I've created a repository in case anyone wants to get this started: https://github.com/fatiando/moulder