rccn/subscriber: Add dependency injection
rccn/subscriber: Add dependency injection

Modified by <keith@rhizomatica.org>

Inject dependencies into the subscriber class to allow for easier unit
testing and more clearly scope the subscriber's external
interface. Right now the parameters are set by default to their global
scope equivalents, so there is no change to the external API.

Rhizomatica/rccn,Rhizomatica/rccn,Rhizomatica/rccn,Rhizomatica/rccn,Rhizomatica/rccn