v0.15.0

• Removed limit of 23 bytes for the client id as in MQTT 3.1.1.
• Removed two possible race conditions for offline messages.
  The race conditions were:
  1. the restoration of all subscriptions was done after connack,
     which
     means that there was a tiny window were a message could be lost.
  2. the puback for a QoS 1 packet was sent before the write was
     concluded.
• Dumped node v0.8 support forever.