Unit test for feed creation

Tests feed object, atom and rss string
Added test cases for all elements and travis support
Added unit test cases for extension loading

Also fixed bugs regarding the upcoming feed.py fix
Fix domain and ttl bugs for RSS generation

In the context of RSS cloud domain was spelled incorrectly as ‚donain‘
which resulted in an error while unit testing. Additionally, also in
RSS ttl (Time to live) wasn’t transformed to a string.
Added dateutil requirement for travis

python-dateutil was missing for a clean install of the module
Fix skipHours for Python 3

The xrange function isn’t available in Python 3. The range function
however is doing the same job and is working for both Python 2 and
Python 3.
Merge branch 'master' of https://github.com/snipem/python-feedgen

This merges in a pull request by snipem brining in some unit tests and
bug fixes:

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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.

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Signed-off-by: Lars Kiesow <lkiesow@uos.de>
Fixing some small test issues

This fixes some small issues with the unit tests and adding a test case
to the Makefile so that all tests can easily be invoked by running:

  make test

Signed-off-by: Lars Kiesow <lkiesow@uos.de>