DOC: New documentation system based on numpydoc

The system is stolen from pandas and the current docs are changed
so that they fit into the new doc system.
DOC: Update gitignore with documentation changes
DOC: Make existing docstrings numpydoc compatible
DOC: Use source instead of installed version
Add ggplot.__version__ and use it in setup & docs
Only load data sets on use and not on import

Use a lazy load proxy for the datasets, which is some kind of hack
(see `ggplot/exampeldata/__init__.py` for the details) and means that
the data sets now need to be explicit imported (and not via *).

Closes: #111