Set up auto-deploy to GitHub pages from master

[domenic]

At @tobie's requests, so he can stop managing merge conflicts between different index.html instances.

This will allow us to stop checking in the build artifacts into the repo. The steps necessary (many of which will need to be done by @heycam) are:

• Sign up for Travis CI and add heycam/webidl to it (see guide)
• Create encrypted credentials file (see guide)
• Update the .travis.yml in this branch to have the appropriate value for ENCRYPTION_LABEL, generated in the previous step
• Add the deploy_key.enc file, generated in that previous step, to this branch
• Create a commit on gh-pages that deletes everything but index.html
• Merge this PR (to master), and monitor travis CI to see if things are going smoothly
• Switch this repo's default branch to master under GitHub's per-repo settings page
• while you're at it, it would be awesome if you could add a description and link to the spec at the top of the repository.

[annevk]

Only two more steps to go here \o/

[heycam]

This is done! Thanks Domenic for the instructions and Tobie for taking me through them!

[annevk]

This is great!