Merge pull request #120 from JanSchulz/fix_unittests

Fix unittests
Add __ALL__ to all relevant files

This makes `from ggplot import *` cleaner, not cluttering the namespace
with other imported names.
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
update .gitignore for docs and pycharm
TST: shorten long running test
Small fixes in manual tests
TST: initial version of geom (baseclass) tests
Add possibility to specify dataframe and aes per geom

This makes it possible to specify `geom(data=df, aes=aes(...))`