Implementing Travis CI integration.

Related to ticket #42

Implements Travis CI integration. @pydanny you need to do the following to activate travis-ci for django-admin2:

* login with GitHub into https://travis-ci.org/ 
* Go to https://travis-ci.org/profile and activate the on/off switch for django-admin2
* merge this pull request into master

That's it. The first built will be triggered by your next push to repo.