Added unit testing and fixed bugs

Hey
##### Unit tests
I've created unit tests for the module, especially for feeds and entries. Whilst entry tests aren't fully implemented yet, unit test cases for feed creation are complete and working. They cover the creation of a full blown feed in Atom and RSS.

##### Bugs fixed
Thanks to the unit test cases, I was able to discover three bugs concerning the fields domain, ttl and skipHours. According to the lastest test results, they are resolved and working with both Python 2 and Python 3.

##### Travis support
Additionally, I've added Travis support. Which is a Continuous Integration platform powering Github. Have a closer look at the Travis builds of my fork:

https://travis-ci.org/snipem/python-feedgen
[![Build Status](https://travis-ci.org/snipem/python-feedgen.svg?branch=master)](https://travis-ci.org/snipem/python-feedgen)