add warning to mpld3.show() in IPython notebook

This attempts to detect if the code is running in an IPython session, and if so prints a warning if ``mpld3.show()`` is used, with a link to the documentation.

Hopefully this will stop the stream of user questions about mpld3 freezing in IPython notebook!  It's not freezing, it's simply opening a web server, as expected with the ``mpld3.show`` command :grinning: