Creates a subscriber binding to the given address and 
    subscribe the given topics.
    The callback is invoked for every message received.

        Args:
            - address: the address to bind the PUB socket to.
            - topics: the topics to subscribe
            - callback: the callback to invoke for every message. Must accept 2 variables - topic and message
            - message_type: the type of message to receive

def subscriber(address,topics,callback,message_type):
    """
    Creates a subscriber binding to the given address and 
    subscribe the given topics.
    The callback is invoked for every message received.

        Args:
            - address: the address to bind the PUB socket to.
            - topics: the topics to subscribe
            - callback: the callback to invoke for every message. Must accept 2 variables - topic and message
            - message_type: the type of message to receive
    """
    return Subscriber(address,topics,callback,message_type)