Structure and consistency of the base persisters.

[thorwhalen]

We need to structure the persisters construction interface more.

Principles:

• Have at least one common interface that is common to all (I believe this is going to be source as the first argument, and filt or subset, or something like that as a second, and after that they could diverge. They could actually share another common "constructor interface" with a uri containing all, or most of what's needed (most information COULD (not saying it SHOULD) be encoded in a uri.

• There should be at least one very specific, very low level constructor. One that is specific to the storage system, and in the language of the tool (say sqlalchemy, pymongo, etc.) that we use to talk to it.

Questions arise as to whether the __init__ should be.
Should it be a lower level kind, specific to the context (but with factories or subclasses offering other, usually easier, ways to make persisters)?
Should it be one of the higher level consistent API ones such as the one including a uri?

[thorwhalen]

Some relevant issues:
#5
#38