Add a gateway

[dkubb]

This branch will add a gateway to the memory adapter. It will also produce a prototype interface for #update on the adapter.

[solnic]

@dkubb @mbj @snusnu I don't think we need to be that specific with rake version :)

[dkubb]

@solnic sorry for the force pushes. Just working on getting the interface right and some of the commit history ended up being garbage. fwiw I usually force force on feature branches, and not master.

[dkubb]

@solnic @mbj @snusu any comments before I merge this in?

@mbj I wanted you to check out the addition of the #[] and #[]= methods to the adapter interface and make sure they make sense to you. The point is that they return a gateway associated with the adapter wrapping a relation. I would want to use this convention in all the adapters if possible.

I realize the adapter is really only communicating with the gateway, so in some ways the interface can vary on an adapter by adapter basis, but I've been trying to find an interface that will work in the general case. From there I can begin to make an abstract adapter as well as an abstract gateway; eventually having one gateway for all adapters. The less code a new adapter author has to write the better; I'll leave it open for them to customize one, but we should have good defaults.

[dkubb]

Even though @solnic and I discussed this approach and would like to change the interface to use mutable materialized relations, I'm happy enough that this is moving in a good direction so I'm going to merge this now.

I'll begin work on updating Axiom::Relation::Materialized to be mutable and for #insert and #delete to propagate the write down to the materialized relation.