Build docs in TravisCI and host on the fatiando/docs repo

Moving away from ReadTheDocs has it's benefits, like being able to install things (no need to mock the imported modules). 

The docs are built using TravisCI and pushed to the `gh-pages` branch of [fatiando/docs](https://github.com/fatiando/docs) when the Travis build is trigger from the `master` branch (i.e., the commit was directly to `master` or the PR has been merged).

Building the docs and checking the links is part of the test suite now.

Fixes #131