um, a bunch of stuff I changed on Friday, but for got to commit so now I have no idea what all of the individual changes were
scoping back implementation to http and https only
passing through max_concurrent and event_listener to downloader construtors
more scope cleanup
removed cancelled callbacks for now
added "prime" (to eventually replace original) download function that limits the maximum number of conncurrent connections as well as utilizes the event listener state machine callbacks