Allow saga's to override correlation logic.

[rhinof]

By default, grabbit correlates a message to the correct saga instance when a handler replies to an incoming message. There are many cases in which you would like to interact with a saga instance and send it messages not in the context of replying to a specific message originating from the saga.

This is an improvement and should provide a better API than the one developed for #172

[rhinof]

Need to reevaluate how to overcome potential race conditions. currently on hold