Track the time of the last survey notification, and only show another one if minimum delay has elapsed

This adds a check when a new survey notification would be displayed, and prevents the notification if less time than MINIMUM_SURVEY_DELAY (currently set to 5 minutes) has elapsed. After creating a notification, the LAST_NOTIFICATION_TIME in local storage is reset to the current time.

This fixes issue #51 .
nit: I am paranoid about operations ordering. could you put parens like (x - y) < z?