DOC: add doc requirements for readthedocs

Also add numpy to require in setup.py at the *last* position
so that installs on readthedocs.org work *before* all other
packages and move statsmodels to the first position
(-> installed last) so that stasmodels can be installed when
ggplots "python setup.py install" is run.
DOC: fix contributing documentation

code in contributing.rst doe snot need to be executed, as it
is exmaples of a new geom. So stop that :-)
todo: mark some of the items as inplemented
DOC: print plots in docstring examples

Also add stat_function to API docs.
DOC: small fix in section heading
Merge pull request #180 from JanSchulz/readthedocs

Add changes to make docs compile on readthedocs