Allow publishing with all options accepted by Bunny

Sneakers currently cherry-picks out a few publishing options to pass to Rabbit, and discards all the rest. This makes it impossible to perform some useful operations, such as adding meta data to a message, or settings its TTL. Since Sneakers goes out of its way to allow workers to ready from the metadata sent by Rabbit, it doesn't makes sense for it to forbid its workers from publishing metadata. This change is backwards compatible with the current Sneakers because it just passes through all the options to Bunny.