Travis integration

This branch provides the functionality to test the validator against Python 2.6 and 2.7 using tox and Travis. I tested this with my own Travis account and confirmed that it works.

I don't know if this means that the STIX validator is fully compliant with Python 2.6, but at the very least, it doesn't blow up on the most simple path.

If the documentation is modified to indicate support for 2.6, then it is only missing a reference to the dependency on argparse (which is available for 2.7 and a backport option for 2.6 via PyPI).

This branch is mostly a starting point for testing to introduce the necessary infrastructure for easily testing against multiple Python versions.