Set up sphinx-gallery

[WardBrian]

Closes #57
A few notes:

• @eickenberg I think it would be good to write a brief introduction to what the goal of the 2d convolution example is, and maybe a summary/conclusion after the final code block.
• Sphinx-gallery produces .ipynb files, which causes nbsphinx to complain about there being two sets of source files for the final page. As a result, I had to disable nbsphinx, and it seems like we'll want an either-or approach here.
• Building the docs fully does now require the full dependency chain to be installed. You can run make fast-* (e.g. make fast-html) to build the docs without running the gallery examples.

[eickenberg]

LGTM!

[eickenberg]

I will improve the example, but this can be merged in the meantime