Add tox and Travis-CI support to repository

Exactly what it says on the tin. This sets things up so that commit pushes are tested by the Travis-CI build service, on multiple versions of Python. Right now, it's only set up to use 2.6 and 2.7, but it probably wouldn't be hard to figure out support for older versions, I just don't know how far back you care about. My own personal motivation is more forwards - I need dispatcher to support Python 3, and I see this as not only generally useful, but an important stepping stone to Python 3 support (which will arrive in a subsequent pull request).

There are a few cosmetic changes. I've made "tests" a submodule of dispatch. I've added a .gitignore file. If any of this is objectionable, let me know so I can adjust my branch accordingly (same if you want to support Python versions farther back than 2.6). My main objective is getting this merged mainline so that I'm clear to work on Python 3 support.

Do also note that you have log into Travis with your github account, and switch on testing for your dispatch repository, before Travis will actually listen for your pushes. It's not enough to just have the .travis.yml file in your repo. Mine is already set up.