Reorganization: moved the adapters into a subdirectory of lib.
Redis-backed attachments now go into a different hash.

So you don't get the attachments when you fetch the object's base
fields, which is the point of thinking about them as attachments.
Attachments are now removed when the object is removed.

This completes the work for issue #6.
Merge pull request #9 from ceejbot/redis-improvements

Fix issue #6 by reworking redis attachments