Six is a module that provides a clean, standardized interface for
handling python2 and python3 from the same code base. This adds a
requirement on six as a build-time dependency, the plan is to use it
only for python generators (those called during build time)

While it certainly is possible to reimplement much of what six does
scratch and not add another python dependency, I think it's better to
just use six. For one thing a large number of python modules already
depend on six, so the chances are good that most people already have it
installed. Second, it's the de facto standard for supporting complex
code bases in 2 and 3, so it's familiar and many of the corner cases
have already been addressed.

This adds the necessary cmake boilerplate to ensure that six is
available. At this time I don't know of a specific version being
required, but I am currently using 1.9.0

v2: - update README with six dependency (Jordan)

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewe...