Activate sphinx-gallery on doc build

[dstansby]

#83 added automatic doc hosting on readthedocs.io, but I had to disable the example gallery building due to xvfb issues. This issues is to track fixing those issues and enabling the example gallery as part of the doc build.

[melissawm]

Ok - I've been taking a look and I think I found it! See #132

Removing qtgallery from the list of extensions did the trick - and then there were a few other sphinx errors that I also addressed but maybe they need to be checked later. Hope this helps!