API fixes

seems the pagerduty API has drifted since this was first written -- not all alerts will result in an incident key being non-nil. the patch just tries to figure out what the best subject is and spit that out if possible.

There's also a note on minimum requirements for IRC consumers of hubot, a problem I ran into while getting it to work.

there are also commits which add features for dealing with open incidents and setting override windows.