Do some (automatic) continuous integration testing

We do have lots of tests (executed with ```python setup.py test```) and we have what I would call integration testing with ```tox``` (testing Python 2.6, 2.7, pypy, 3.1 and 3.2).
That is nice already.
However, not everybody has all these versions installed.

So I suggest using [travis-ci](https://travis-ci.org/JonnyJD/python-musicbrainz-ngs) for every branch (except known old branches and gh-pages) and every pull-request. This will make sure that the tests run on every supported version and everyone that could be blamed receives an email.

On the other hand everybody should do at least some testing locally before pushing commits, obviously. Using tox for this is nice, but everybody has other Python versions installed, most people will have only one or two. Maybe renaming tox.ini to tox-default.ini would be the best.
So everybody can decide what to put in this config.
Having red text (for versions not found) should not be normal, because then you might overlook real problems.

Testing with 1 version locally should be mandatory, but testing with more than one Python 2 and one Python 3 normally doesn't help much.