Added intro, single-source tools and "What's in Which Python" sections.

These were the separate commits that conflicted each other in earlier PRs. I've combined them into one PR (on an updated master) so there should be no conflicts.
Yes - that's a useful utility for updating *existing* code to run on both Python 2 and 3 (as is the futurize command provided with the future module).

Future is also a better choice for anyone converting *Python 3* code to support Python 2, since it has a "pasteurize" script to go in that direction.