DOC : merged offsetbox.rst into artists_api.rst
DOC : added examples index to contents page

I assume there was a reason for only putting the
examples in the html docs, but I don't see it.
DOC : removed out-dated example

Removed example of how parse output from svn logs.
DOC : fixed section titles

Parameter -> Parameters

Example -> Examples
DOC : fixed section hierarchy
DOC : fixed rst formatting
DOC : commented out non-existent file
DOC : updated mpl_toolkits

Added section on high-level plotting with seaborn and ggplot.

Added mpldatacursor and prettyplotlib.
DOC : fixed typo
DOC : updated seaborn blurb

 - use blurb suggest by @mwaskom
 - added links
Merge pull request #3133 from tacaswell/doc_cleanup

General documentation cleanup and reference to spin-off projects.
converted assert into exception
Exception type modified according to @pelson comment
removed assert in `draw_artist` and `redraw_in_frame`
most asserts modified to ValueError
All asserts substituted by ValueError
method name explicitly written in the message
Most asserts changed to ValueErrors. Two changed to warning
c* files done
assert removed