Add support for atompub dup handling.

Some atompub servers (e.g. AtomHopper) will return an http 409 code
on posting of an entity with a duplicate message ID. this prevents
them from getting the same entity twice. Yagi should *not* try to
repost the entity, as they already have it.
Merge pull request #10 from rackerlabs/atompub_dup_conflict

Add support for atompub dup handling.