Beginning transition away from fixed refresh intervals

PollService:
-nextEvent() returns the first event in the next 24 hrs matching user preferences (free time & availability)
-onHandleIntent() checks whether an event is active based on the returned events begin time, schedules a wake at the events end time.
-takes into account buffer minutes for begin
BUG: does not use buffer minutes with end time

PollServiceReceiver:
-renamed to BootCompletedReceiver