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 logger from gravmag_harverster.ipynb
Merge pull request #139 from hbueno/rm-logger

Removed logger from gravmag_harverster.ipynb
REMEMBER: README must be rst for PyPI

PyPI doesn't render markdown content. The readme must be in RST for the
package page in PyPI.
Set the logo image as a title

Using rst alias for this.

http://stackoverflow.com/a/19189518
Rename readme extension to rst
Merge pull request #142 from fatiando/readme

README must be reStructuredText instead of Markdown
PR checklist as an h2 in markdown
New PR checklist item: check for new dependencies
More sections on devel guide

Getting started: sign up for Github, make a fork, etc
Setting up: install things, env variables
Building: compiling C ext and Cython
Testing: unit and doctests
Code style: PEP8 and the auto checker
Documentation: Building and serving the docs

All have links to reference material (most are software carpentry)
Fix broken links to the mailing list
Link to github guides page

They some really nice tutorials.
Separate branching into a new section

Added some more tips for getting started: look at issues, tell what
you're working on, low-hanging fruit.
Mv docstring convs to Documentation

Reorder some things in the PR section as well.
Merge branch 'master' into devel-guide
dens arg for talwani.gz

Overwrites the density property of the polygons. Used for making
sensitivity matrices (set density to 1).
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