Documentation building and some doc additions

Try to add some documentation build: mostly reused from pandas.

*  Adding three new sections to contributing: "How to contribute via pull requests", "how to add unittests" and "some git commands"
*  add `__ALL__` property to each python file to declutter the namespace after a `from ggplot import *`

**Open Issues**:

- [x] decide on one dialect for docstrings -> numpydocs
- [x] check some errors in the current doc strings
- [x] find out why ggplot.* methods are not added to the docs -> it seems that this was a problem with the order the sphinxext are set in conf.py -> all problems with this are gone
- [x] fix the build warnings -> see above
- [x] add a ggplot.__version__, which is then used in docs/make.py and also in setup.py

-> Ready to merge :-)