Use timestamp to check for new questions

[Rich Trott]

A question_id can vanish from the list if it is closed or deleted. At least, that appears to be why sometimes stackwatch opens 30 tabs. We should check a timestamp instead of or in addition to the question_id. Alternatively or in addition, we can keep track of more than just the last question_id but instead a series of seen-questions. Probably would want to cap it at 50 or so.