Minor tweaks to the docs. Fixed all the warnings and enabled "Fail on warning" option so that problems with the docs will cause the build to fail on warning, the docs are also built for every commit and PR.
Restructured the travis script/after_success so that the docs are only pushed if the build was successful (only master branch and no PRs).
Uses the Python 3.5 build to generate the docs.
Then, if there are any changes, it pushes them
to the streamlink.github.io repo using a key
pair. The deploy-key.enc file here is the private
key of that pair. The public key was added to the
streamlink.github.io repo as a deploy key.
