Added button to notification
strings.xml:
-renaming from 'free time' to 'available' to match default android calendar naming

PollService.java:
-notification of ongoing event now includes a button to either skip (ignore) the current event, or to re-enable the event if it has been skipped.
-added rudimentary support for updating the listview in the main activity from the service using broadcast receiver

MainActivity.java:
-added rudimentary support for updating the listview when receiving a broadcast message

ResetEventService.java:
-simple service to reset the ringer state of a calendar event then restart the poll service

SkipEventService.java:
-simple service to change the ringer of the current event to UNKNOWN, then restart the poll service