[core] Make sure callables without rate limit will never get rate lim…

…ited

Fixes #239
If func.rate is zero or None (as None > 0 == False), don't bother going into
the rate limiting code. This prevents the race condition described by @Venti-
in issue #239

willie/bot.py

@@ -731,6 +731,7 @@ def call(self, func, origin, willie, trigger):
 
         if not trigger.admin and \
                 not func.unblockable and \
+                func.rate > 0 and \
                 func in self.times[nick]:
             timediff = time.time() - self.times[nick][func]
             if timediff < func.rate: