Add support for in-memory adapter.

I'm working on a [gem](http://github.com/boochtek/virtus-perpetuity) to integrate Virtus and Perpetuity, to DRY up the declaration of attributes in 2 locations. I'd really like an in-memory database to make testing that simpler. So I created a [`perpetuity-memory`](http://github.com/boochtek/perpetuity-memory) gem, based on PR #20 by @cgarvis.

But that gem won't work without this PR. Ideally, adapters should register themselves with Perpetuity, eliminating the need for the hard-coded list of adapters. But that seems a bit of overkill to get what I need, and would require changes to the other adapters.

(There should be an emoji for yak shaving.)