Add HTSPSession.monitor()

Other:
 * HTSPChannel.now, HTSPChannel.next properties
 * HTSPDVREntry.start_extra HTSPDVREntry.stop_extra properties
 * HTSPEvent.duration property
 * __str__() on some classes
 * doc strings
 * support for HTSP channelUpdate message
 * support for HTSP autorecEntryUpdate message