Add Travis CI for the pytumblr client

Issue: https://github.com/tumblr/pytumblr/issues/17

After adding `.travis.yml`, the Travis service hook for the repository needs to be turned on (this can be done at travis-ci.org).

![screen shot 2013-08-18 at 1 48 02 pm](https://f.cloud.github.com/assets/164822/982127/b42a1898-07ca-11e3-89e8-7a5795fa62aa.jpg)