Rebuild gh-pages branch on each push

[blazeu]

Make gh-pages branch in this repo be rebuilt automatically from master on each push to master using Travis CI.

On a fork, the build should not push to upstream gh-pages, instead it should do a push to gh-pages branch on the fork.

A push to a branch other than master on a fork can overwrite the gh-pages, but that would still be a bit messy. Instead write it into gh-pages/<branch_name>. Then, the fork owner can set which branch is published in the repo settings on GitHub.

Additionally, on each pull request, Travis should build the commit on that pull request without overwriting the gh-pages.

difficulty/medium initiatives/gci