Refactor Perpetuity::Configuration.adapters to contain classes.

We were using strings before, because adapter classes may not exist.
We'll check to ensure they're defined instead, before adding them.
Add Perpetuity.register_adapter.

Adapters should register themselves with Perpetuity:

  Perpetuity.register_adapter :memory => Perpetuity::Memory
Move registration of "standard" adapters.

These are adapters written prior to the registration API, so their
current versions don't know that they need to be registered. So
we'll register them automatically. We can update the adapters to
register themselves, as there's no harm in re-registering an
adapter.
Add specs for trying to re-register an adapter.
Merge pull request #38 from boochtek/adapter-registration

Add API to register adapters