Removed plotly subpackage from mplexporter package.

Implementation for plotly can now be found at:

https://github.com/mpld3/matplotlylib
Removed import for plotly package from renderer.__init__.py.
Removed mplexporter.renderers.plotly submodule from setup.py.
Merge pull request #11 from theengineear/master

Remove plotly from mplexporter.