Automate documentation build

[ketch]

We keep running into issues with the documentation, and I think most of them could be avoided if we had an automated build that updated clawpack.github.io. Also, it might be helpful to have some tests, though I haven't thought about it much. At a minimum, we should do

make doctest

[ketch]

Even better: we could have Travis build and push our docs! Here's one approach:

http://blog.teracy.com/2013/08/09/automatic-sphinx-docs-deployment-with-sphinx-deployment/

One stumbling block is that we need to be able to build the codes and run all examples for the galleries. But maybe we could have an option to not update the gallery, and do that manually when necessary.