mpl refactor and subsequent mplexporter integration.

This PR supersede the initial work in #558 y the initial refactor branch in #580.
Up to now, the exporter works with all the previous mpl, seaborn, ang ggplot examples, except for the subplots plot (working on it yet).

Our new `BokehRenderer` class supported by mplexporter is a sort of Frankenstein, because the mplexporter support thing we don't, and we support things that mplexporter does not... so, I am using the best of both world whereas we evolved to a more unified solution.

Still work in progress, because I have to use more mplexporter features, but very near to be completed, at least in a first implementation.