def subscribe(topicList):
Abstract
========

Send a SUBSCRIBE control packet.

Description
===========

The SUBSCRIBE Packet is sent from the Client to the Server to create 
one or more Subscriptions. Each Subscription registers a Client's 
interest in one or more Topics.

Signature
=========

@param topicList: list of tuples C{(topic, QoS)}. Each topic is
    an UTF-8 string. 0 <= QoS <= 3
@return: a Deferred, with an extra C{msgId} attribute which you can 
    use to keep track of requests. 
    The callbacks will be invoked with a list of granted tuples (granted qos, failure flag)
    where False in the failure flag means operation Ok)