Cleanup and Travis CI build added

* The `runnable demos` folder has been removed to save some space. #56
* A Travis CI build has been added for the `gdx2d-desktop` library. Travis must be setup for the original repository and the link updated in the README file.
IMHO, it would be nice to move this repository to the [hevs-isi](https://github.com/hevs-isi) Github organisation. #32