Add Travis CI configuration file

Travis CI is an automatic test build service that integrates nicely with GitHub: By default, all commits and [pull requests][1] are tested.  The output can be viewed [online][2] (follow the "Job" links for the individual OTP releases to see [the][3] [details][4]), and pull requests are [annotated][5] with the test results.  That's quite useful for weeding out contributions that break `make test` or don't even compile.

Automatic test builds can be enabled like this:

1. On <https://travis-ci.org/>, use the "Sign in with GitHub" button.
2. On <https://travis-ci.org/profile>, flip the switch to "on" for the `processone/ejabberd` repository.
3. Merge this pull request.

If you like, you could enable [email or other notifications][6] in the [.travis.yml file][7], or you could modify the [list of OTP releases][8] to build against.  The [`--disable-odbc`][9] flag could probably be removed [soon][10].

[1]: http://docs.travis-ci.com/user/pull-requests/
[2]: https://travis-ci.org/weiss/ejabberd
[3]: https://travis-ci.org/weiss/ejabberd/jobs/24484670
[4]: https://travis-ci.org/weiss/ejabberd/jobs/24484671
[5]: http://blog.travis-ci.com/2012-09-04-pull-requests-just-got-even-more-awesome/
[6]: http://docs.travis-ci.com/user/notifications/
[7]: http://docs.travis-ci.com/user/build-configuration/
[8]: https://github.com/weiss/ejabberd/blob/add-travis-support/.travis.yml#L3
[9]: https://github.com/weiss/ejabberd/blob/add-travis-support/.travis.yml#L23
[10]: https://github.com/travis-ci/travis-cookbooks/pull/310