Add Travis CI

At the moment this only runs the simple pp/tests.py, and I changed that to be Py3 compatible.

More robust tests can be added to the `script:` section of .travis.yml.

Runs on Python versions 2.6, 2.7, 3.2, 3.3 and PyPy. Travis will be adding 3.4 soon (see https://github.com/travis-ci/travis-ci/issues/1989#issuecomment-39315535).

To help with #8, so you can quickly see if a commit passes for each Python version.

Enable Travis CI for your repo first at: https://travis-ci.org/profile