Add travis configuration with pypy support

Added a travis configuration file along with an installation script (taken from https://github.com/twisted/klein/blob/master/.travis/install.sh) to install the correct version of pypy.

Fixes #5 
All I could find was https://github.com/travis-ci/travis-ci/issues/1106 which was closed late last year.

I added that script because it was used in klein which is listed as a template for travis built python projects. http://docs.travis-ci.com/user/languages/python/

The comment at the top of the script says it was copied from cryptography, but their install.sh is now different and they seem to have reverted the installation of a custom pypy and then put it back to support mac osx (which I don't really understand)
 * https://github.com/pyca/cryptography/commits/master/.travis/install.sh

So I'll just remove it for now. I note that you're not doing anything like this in machinist or eliot so I guess the default travis pypy is good enough.