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 
I've become a big, big fan of explicit string literals in yaml files.  `language: python` isn't actually ambiguous but the syntax is definitely a trap.  `language: "python"` is much less treacherous.