Move attachment code into couch_att
Move attachment code into couch_att

This is an attempt to isolate the attachment record and
some related code. This will allow seamless upgrades
over time.

fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb