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.
Added Configuration class

This will allow us to test the intitialization of configuration
Rename "Config" to "CONFIG"

It's a near universal convention in Ruby that constants are all upper
case, with PascalCase resevered for Class and Module names. Having the
configuration information saved in a constant with Pascal case makes the
code less clear to programmers who are used to this convention.
Extend self rather than wrapping methods in block