Added support for streaming messages to a process

Implemented #8 
I would usually prefer the ``gen_event`` approach where instead of sending a message to a pid, you call a function that was either provided when the request started or specified as a ``-callback`` on a behaviour implemented by the module that was passed as a parameter. In any case, if you're using message passing, you should (at least) _tag_ your message (e.g. ``{shotgun_data, Data}``) and provide all the possible information. Keep in mind that the other process may receive messages from other sources while it's listening to yours.