New Moulder GUI with more interaction

Allows editing points, setting densities, selecting/deleting polygons
and points, etc.
Few plotting bugfixes

Reload the predicted data from file as well.
Set animated=False before saving so that all polygons are plotted on
reload.
Pickle vertices instead of matplotlib polygon

Was having trouble under windows to pickle the
matplotlib.patches.Polygon. So storing the vertices in the pickle and
recreating the polygons and lines in __init__.
Removed the fatiando.gui package

Moved the Moulder class to gravmag.interactive. Deleted the other
classes and their respective cookbook recipes.

Updated docs accordingly.
Added screenshot of Moulder in its docs
Travis needs to install IPython now
Add IPython as a dependency
Merge branch 'master' into new-moulder

Conflicts:
	README.md
Remove interactive from coverage report
Changelog entry for new Moulder and rm gui pkg
Fix model out of plot error when resizing the plot

When the plot window was resized during modeling, the 'plot'  method was
drawing polygons out of the modelax borders. The same was happening when
calling 'run' again.
Fixed by recreating the polygon and line objects on figure_setup.
Merge branch 'master' into new-moulder

Conflicts:
	doc/changelog.rst
Merge pull request #143 from fatiando/new-moulder

Rewrote the Moulder class for interactive 2D gravity modeling