add submodule commands to the setup script

mpld3 version 0.1+ uses ``git submodule`` to bundle [mplexporter](http://github.com/mpld3/mplexporter). This PR adds commands to the setup.py script to automatically update the submodule when necessary (the 0.1 release requires use of the makefile to correctly bundle the mplexporter source; this has no such requirement).

See the updates to the README file to see how this works in practice.