Python 2.6 tests

This patch is making the tests runnable with python 2.6 as well and enables them on Travis. It's now building on 2.6, 2.7, 3.2 and 3.3. I'd retry a push request explaining that we are using PyLD on a RHEL-based environement.

[![Build Status](https://travis-ci.org/greut/pyld.png?branch=python2.6)](https://travis-ci.org/greut/pyld)