Changed the way mplexporter does legend support.

While crawling an axes object, the exporter would make calls to its
renderer object when need be.

NOW, exporter has a draw_legend method that stores all the necessary
calls to the renderer object in three categories: `line_calls`,
`path_calls`, and `text_calls`.

`get_return` arguments were added to: `draw_line`, `draw_patch`, and
`draw_text` in the exporter. This basically makes a dictionary of all of
the keyword arguments that are required for the call and returns this
dictionary INSTEAD of calling the renderer.

A new method, `draw_legend` was included in the base class which takes
the above-mentioned `line_calls`, `path_calls`, and `text_calls`
arguments. It goes through each call and makes an appropriate call to
its own `draw_line`, `draw_path`, and `draw_text` methods.

This checks out OK with nosetests.

Result: high-level renderers (like plotly) can deal with the legend as a
single entity all at once or choose to ignore these calls by
reimplementing the `base.Renderer.draw_legend` method.