Python 3 compatibility

[ivan-krukov]

This is the resolution for #17.

I had to change the way the module is imported. Now instead of

Rscript bin/make_ggplot2_functions.R >> pygg/pygg.py, one would

Rscript bin/make_ggplot2_functions.R > pygg/ggplot2_functions.py

I think this is a little cleaner. It should also probably be added into the build script.

The functions are imported in __init__.py through a series of exec statements given the content of pygg/ggplot2_functions.py.

All of the above is to allows to cleanly import everything through __all__ without having to do from pygg import *, which would not work in py3.

The rest of the changes are of a cosmetic nature (print -> print(), etc.)

Thanks for a great module!

[sirrice]

this is awesome! will take a look in the next day.