Travis support

This add [Travis](https://travis-ci.org/) support which target all the 4 major GHC versions.
A bit of manual tweaking might be required, as I have simply ported the travis.yml file from
my [Mandrill](http://github.com/adinapoli/mandrill) library.

It will probably be necessary for you, @chrra (or whoever the owner of this repo is) to sign-in in Travis.

Alfredo