Add PlotElement base object
make Toolbar, Button, and Axes derive from PlotElement
make Axis derive from PlotElement
make Grid inherit from PlotElement
make Lines, Paths, Markers inherit from PlotElement
make PathCollection inherit from PlotElement
make Image and Text inherit from PlotElement
make plugins inherit from PlotElement; add deprecation warning to process_props
some cleanup of unused functions
make mpld3.Line derive from mpld3.Path
BUG: fix null path logic
add null paths to test_fill
clean up coordinates code
make Markers inherit from PathCollection
Merge pull request #98 from jakevdp/object-refactor

MRG: refactor the mpld3 object structure