Change `draw_line` and `draw_markers` to 'draw_marked_line`.

Exporter lets the base renderer farm out the appropriate calls to
`draw_markers` and `draw_lines`. Renderer's can reimplement the
`draw_marked_line` method if they wish to keep the line and marker
information bundled together.

If a renderer doesn't wish to do so, they should NOT reimplement the
`draw_marked_line` function, instead reimplementing both `draw_line` and
`draw_markers`.
Removed `lines` and `markers` variables from draw_line in exporter.

If there is no `dasharray` for a Line2D obj, the `linestyle` dict is set
to `None`. Same goes when `marker` is not populated.

This way, the `base` renderer uses `linestyle` and `markerstyle` in
`draw_marked_line` to decide which or both of the `draw_line` and
`draw_markers` methods to call.
Changed: dsharray - dasharray.
Added `label` to all renderer's `draw_marker` and `draw_line` calls.

Running `nosetests -w tests` checks out now.
Merge pull request #15 from theengineear/marked_line_2

Change `draw_line` and `draw_markers` to 'draw_marked_line`.