Add Python 2 backwards compatibility

[andrewsg]

In order to facilitate smooth migration from GAE Python 2.7, we should add backwards-compatibility with Python 2. Ideally, users will be able to convert to using Cloud NDB in-place in their GAE Py2.7 applications, and then upgrade to Python 3 without further NDB changes.

[safetymonkey]

I realize there's probably not a concrete roadmap or committed dates for these sort of things, but do you have any sense for when this feature might be achieved? As you might imagine, this would be a huge simplifier for Py3 migrations.

[cguardia]

Hi, thanks for your interest. We are hoping to have this working at some point in November.

[cguardia]

Python 2 compatibility is now merged into master (via #203).