Add the ability to specify the exchange type

I've extended the RabbitMQ target to now support specifying the exchange type. (My project required this as our exchanges are defined at direct, not topic.)

I see this as just giving the user more control, and not forcing them to use a specific type of Exchange. (It also allows you to use things like the riak-exchange project, which defines an x-riak exchange type)

It's a pretty straightforward change.