Add Travis CI

[jpadilla]

It'd be awesome to have this repo setup with Travis CI to automatically run tests. Tests will run on Pull Requests as well letting you know if they passed tests and are safe to merge. Having their little badge in the README really gives confidence to other developers.

Something like this should work:

language: python

python:
  - '2.7'
  - '3.3'

install:
  - pip install -r requirements.txt

script:
  - python slugify/tests.py