Fix 'setup.py test' command and add .travis.yml file

This fixes the use of the deprecated `execute_manager` (not available in Django 1.3 and 1.6+).

It also adds support for Travis -- if you haven't heard of Travis, check out http://about.travis-ci.org/docs/

You can then add this cool build badge to your README file: [![Build Status](https://travis-ci.org/jonashaag/django-celery-email.png?branch=master)](https://travis-ci.org/jonashaag/django-celery-email)