Added support for Redis PubSub:

Added support for Redis PubSub:

 * support commands SUBSCRIBE, UNSUBSCRIBE, PSUBSCRIBE, and PUNSUBSCRIBE:
   * PUBLISH is a regular Redis command, already supported.
 * last argument of subscription changes commands must be a callback, a
   coderef:
   * for each message, the callback will be called with three parameters:
     * the messages data;
     * the topic to whom the message was sent;
     * the subscription that catched this message.
 * you can subscribe to multiple topics or topic matches with the same
   callback: messages to any of those topics will invoke the callback;
 * you can call p?subscribe() multiple times with the same topics or
   topic matching rules, and with different callbacks:
   * we will manage a single Redis subscription, and call all the
     callbacks per message received.
 * p?unsubscribe() calls require the original callback, to remove the
   correct subscription.

Signed-off-by: Pedro Melo <melo@simplicidade.org>