Allow publishing with all options accepted by Bunny

Previously Sneakers cherry-picked out a few options to pass to Rabbit,
and discarded 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.
Merge pull request #53 from flexminder/allow_all_publishing_options

Allow publishing with all options accepted by Bunny