Return list of service alerts ('bulletins') for a route or stop.
        
        Arguments:
            `rt`: route designator
            or
            `stpid`: bus stop number
            or (`rt` and `rtdir`) or (`rt` and `rtdir` and `stpid`)
        
        Response:
             `sb`: (bulletin container) contains list of
                 `nm`: bulletin name/ID
                 `sbj`: bulletin subject
                 `dtl`: full text and/or
                 `brf`: short text
                 `prty`: priority (high, medium, low) 
                 `srvc`: (routes bulletin applies to) contains list of 
                     `rt`: route designator
                     `rtdir`: route direction
                     `stpid`: bus stop ID number
                     `stpnm`: bus stop name
        
        http://realtime.portauthority.org/bustime/apidoc/v1/main.jsp?section=serviceBulletins.jsp

def bulletins(self, rt="", rtdir="", stpid=""):
        """
        Return list of service alerts ('bulletins') for a route or stop.
        
        Arguments:
            `rt`: route designator
            or
            `stpid`: bus stop number
            or (`rt` and `rtdir`) or (`rt` and `rtdir` and `stpid`)
        
        Response:
             `sb`: (bulletin container) contains list of
                 `nm`: bulletin name/ID
                 `sbj`: bulletin subject
                 `dtl`: full text and/or
                 `brf`: short text
                 `prty`: priority (high, medium, low) 
                 `srvc`: (routes bulletin applies to) contains list of 
                     `rt`: route designator
                     `rtdir`: route direction
                     `stpid`: bus stop ID number
                     `stpnm`: bus stop name
        
        http://realtime.portauthority.org/bustime/apidoc/v1/main.jsp?section=serviceBulletins.jsp
        """
        
        if not (rt or stpid) or (rtdir and not (rt or stpid)):
            raise ValueError("You must specify a parameter.")   

        if listlike(stpid): stpid = ",".join(stpid)
        if listlike(rt): rt = ",".join(rt)
        
        url = self.endpoint('BULLETINS', dict(rt=rt, rtdir=rtdir, stpid=stpid))    
        return self.response(url)