Make documentation runnable on binder
I was playing around with a getting our docs runnable on binder today.

The tldr is that you can run them at https://mybinder.org/v2/gh/TomAugspurger/pandas-binder/master?filepath=build%2Fjupyter%2Fgetting_started%2F10min.ipynb

The source is at https://github.com/TomAugspurger/pandas-binder.

---

Overall, I think this is neat. It uses sphinxcontrib-jupyter to turn our (lightly modified) ReST files to Jupyter notebooks.

There are a few changes to the pandas source that I'd like to propose

1. Adding a little boilerplate to the user guide and getting started pages to link to the runnable version (see https://github.com/dask/dask-examples/blob/d7a69122d9c1c24f6583e7d46cd089cf1eb70ae8/conf.py#L49)
2. Maybe restructuring our jinja `{{ header }}` to be an explicit code block in each page (it doesn't seem to play well with `sphinxcontrib-jupyter`). I can probably make a workaround though.

Lots of things are probably broken (e.g. refs like :ref:`DataFrame`) don't resolve correctly. But I'm happy with this as a work in progress.