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.
While is a technically accurate description of the differences between the two, I suspect it would create misleading impressions for readers. The key difference is that six supports all the way back to Python 2.5, while future only goes back to Python 2.6. This means that future relies more heavily on the syntactic features and future imports added in Python 2.6 to achieve single source compatibility. This can result in cleaner single source code for projects that don't need Python 2.5 compatibility.