Integrate with Travis CI

Using these commits you can hook up the main Gallery3 repo with [Travis CI](https://travis-ci.org/). After the connection has been made, Travis will automatically run unit tests on all new commits pushed to the repo. It will also [mark each Pull Request](http://about.travis-ci.org/blog/2012-09-04-pull-requests-just-got-even-more-awesome/) with the results of the tests.

Here's what this setup would look like on Travis:
https://travis-ci.org/jozefs/gallery3/builds/5237737

If this is merged, the build script in gallery3-packaging will need to be updated to remove the Travis .yml file.

If we convert the README file to Markdown, we'd be able to put a badge indicating whether the tip of the master branch has been built successfully, like this: [![Build Status](https://travis-ci.org/jozefs/gallery3.png?branch=travis)](https://travis-ci.org/jozefs/gallery3)

We can make the tests run on several different versions of PHP. 5.3, 5.4 and 5.5 are supported. The current setup runs on 5.3 only.