Add Travis configuration file.

This adds a `.travis.yml` to the repository, allowing forkers (and the project itself!) to let [Travis](https://travis-ci.org/) run the tests against the pushed code and pull requests.

Example of Travis integration with Picard: https://travis-ci.org/Freso/picard

Some notes:
* It's only testing Python 2.7. I wanted to have both 2.6 and 2.7 in the testing matrix, but since it's *much* more manageable to install PyQt4 via `apt-get`, I opted for that instead of having to build it "manually" for Py 2.6. If it's really desired, it can be added later.