add ModelChain tutorial, nbsphinx extension

[wholmgren]

This PR adds a ModelChain tutorial to the documentation.

I'm trying a different documentation build technique than I've used in the past: write the tutorial in a Jupyter notebook, then use nbsphinx to embed it in the html documentation. After a few hiccups, I found that overall it's a lot easier than writing rst files with ipython directives. The downside is that the source ipynb file is harder to read with a simple text editor (or on github) than a simple rst file. The ipynb file doesn't have any output, though, so it's not as bad as it can be.

You can few the documentation here:

http://wholmgren-pvlib-python-new.readthedocs.io/en/mcdocs2/modelchain.html

I'll leave this open for a day for edits/suggestions. We can always improve it later, too.