[CMake] Finding Python3

[Branan Riley]

I've mad a very cursory effort to add Python 3 support to CMake. All
I've done so far is take FindPythonLibs and FindPythonInterp, and
change the variable names and the python versions. It doesn't yet have
framework support on Mac, as I have no idea how to specify a Python 3
framework. Besides that, the code should probably be adjusted so it's
not duplicated across two modules - a FindPythonLibsCommon.cmake file
or something that does all the "hard" work.

I'm making this available now so people can play with it and give
feedback. I'll clean up the code and get another message out when I
think it's in a state it can actually be merged into the main CMake
git tree.

For now, my work is in a clone on github at
http://github.com/branan/CMake/tree/python3 . You can add my work as a
tracking branch to a checkout of the main CMake git tree with the
following commands:

git remote add -t python3 branan git://github.com/branan/CMake.git
git fetch branan
git checkout -b python3 branan/python3