ENH: add local keyword to notebook tools

I'm at PyCon and there is very spotty wireless. I wanted a more streamlined way to run mpld3 in a notebook without a web connection. This PR adds that. Essentially, you can now call
``` python
mpld3.enable_notebook(local=True)
```
and plots in IPython will use a local version of the mpld3 and d3 javascript libraries.