WIP: create a sphinx website

This is an inital PR containing a sphinx website which will host documentation and examples for mpld3.

All the content is in the ``doc`` directory.  I also wrote a sphinx extension that automatically generates examples from code in the ``mpld3/examples`` directory.

To preview this, make sure you have sphinx installed, then do

```
[~]$ cd doc/
[~]$ make html
[~]$ open _build/html/index.html
```

Still some work to be done here, but I think this is a good start