Travis CI

I integrated Travis CI as a replacement to executing "python test.py" when submitting a pull request.

It should be merged at the same time with Pull Request https://github.com/Instagram/python-instagram/pull/98

It would be possible to test the OAuth Client ID and Client Secret if these were environment variables, as per Pull Request https://github.com/Instagram/python-instagram/pull/95  and configured as http://docs.travis-ci.com/user/environment-variables/#Secure-Variables 

I omitted http://docs.travis-ci.com/user/database-setup/#Redis as there are no unit tests for sample_app.py yet.

Also http://docs.travis-ci.com/user/status-images/ will need to be amended to reflect the git branches of https://github.com/Instagram/python-instagram rather than my forked repository.

"python tests.py" output below (yes this would be redundant if this pull request is merged):
```
..........................
----------------------------------------------------------------------
Ran 26 tests in 0.385s

OK
```

I've completed the Contributor License Agreement ("CLA") too.