mpl refactor

This is a full refactor of our mpl support to help me add the mplexporter from Jake.
`axes2plot` is now more compatible with the current generations of plots.
I added a `mpl_helper.py` to throw all the helper functionalities.
I also refactored the functions generating the axes, grids, lines, markers and collections, splitting them in functions which generates the glyphs and objects, and other functions to setup the properties for each one of them.
Finally, I fixed some bugs, corrected and tested all the examples (mpl, seaborn and ggplot).
The work with the Jake's mplexporter will be pushed in another PR based in this one...

Good weekend!