Py3k Compatibility, and Travis CI testing

Hi Again!

This fixes some py3k things, mostly in the crowdserverstub.py. I've also set up travis-ci to run the tests against Python 2.7 and 3.3 after every push.

If you don't have a travis account yet, here are roughly the steps to set it up on this repo:

1. Follow the first two steps at http://about.travis-ci.org/docs/user/getting-started/
2. Merge this pull request to get the .travis.yml file
3. Watch travis build and test the repo
4. Add the travis build status badge to the README

I set it up on my branch to make sure it all works: https://travis-ci.org/lukecyca/python-crowd

BTW: I've got a new package which depends on python-crowd: https://github.com/lukecyca/pyramid_crowd.  I'm also considering making an crowd authentication module for Sentry.