Add a travis-ci info file for Travis CI

I like TeamCity and think we should definitely keep it.  Travis is no replacement for it, but it has the nice feature of notating pull requests.  TeamCity could do that too, but for now it doesn't, so there's no reason we can't use both.

Things Travis CI does not do well:
 * Single platform.  You can pick Mac (which I did not try) or Linux, and then you're stuck with it.
 * Doesn't support Windows at all afaict.
 * Build has to download the entire NDK every time (ouch.)
 * Doesn't seem to do "granular" failures, e.g. x/y tests passing.

The benefits are:
 * It will attempt to build any submitted pull request and [note if it would break the build] (http://about.travis-ci.org/blog/2012-09-04-pull-requests-just-got-even-more-awesome/).
 * Can build multiple targets simultaneously (e.g. probably could add Qt.)
 * Sends an email by default if the build gets broken.

Anyway, if you want to use it, login at https://travis-ci.org/ and enable the repo after accepting this pull.  It'll request permissions through GitHub to update the pull request statuses.

-[Unknown]