Added more documentation and made some name changes for readability.

All of these changes are related to either documentation, improved
readability, or adhering to PEP8 guidelines.
Added a verbose=False default to fig_to_plotly.

The PlotlyRenderer now has a msg attribute that tracks the process of
rendering a figure from mpl. When verbose=True, it will be printed. If
not using fig_to_plotly, you can access it normally through
renderer.msg.
Added spaces to PlotlyRenderer.msg. Better readability.
Set default 'hovermode' to 'closest'. Looks a bit nicer I think.
Tiny commit, just edited an docstring for open_figure().
Added draw_marked_line to PlotlyRenderer.

The mpl Line2D object allows users to create ONE line with both lines
and markers. Because plotly allows this too, we don't have to draw lines
AND markers as separate traces.
Updated mplexporter.

This update includes the addition of a draw_marked_line which aggregates
the drawing of lines and markers when they refer to the same data.
Updated renderer to deal with `draw_marked_line`. Updated tests.

This commit brings the rest of the matplotlylib project up to speed with
the mplexporter update that was just committed.

Now mpl Line2D objects that have markers and lines associated with them
can be treated as a single object by renderers, if desired.

Nosetests required updating to deal with the new syntax. On that note,
`label` and `size` were added to data dictionaries.
Added nosetests for bar charts and scatter plots.
Quick update to README.