Disallow negative time extension

Otherwise anyone can just end a vote.

models.py

@@ -57,6 +57,10 @@ def register_vote(self,choice,member=None):
 		return was_changed
 
 	def extend_duration(self,hours):
+		if hours <= 0:
+			logging.info('Member %s attempted to extend negative time' %
+				(member.nickname(),users.get_current_user()))
+			raise Exception('Negative extensions are not valid.')
 		self.duration += hours
 		if self.start_time:
 			self.end_time = self.start_time + timedelta(hours=self.duration)