Split out some driver-specific abstractions

[kgaughan]

It would be nice to be able to take advantage of driver-specific features, and provide fallbacks where they're not supported. These include things like:

• nested transactions (with the current non-nested transaction emulation acting as a fallback),
• upserts,
• ...and potentially a host of other features.

The first step would be to create some class that would provide a basic implementation of these things, with some way for other abstractions to be registered on a driver-by-driver basis, possibly using a setuptools-based hook.