v0.23.0

• Add Client.sendString(headers, body, callback)
• Add Client.ack and Client.nack methods
• Fix MemorySocket close test under Node 4
• Fix qs string parsing bug that caused a string value prefixed with digits to be interpreted as a number
• Modify: Close all owned channels on pool close
• Modify: The parameter of 'client-individual' ack function can either be a callback function or a sentOptions object
• Modify: message.ack and message.nack are no-op functions on an auto mode subscription
• Modify: Calling message.ack is now optional for an 'auto' mode subscription
• Deprecate use of message.ack and message.nack methods (replaced by client.ack and client.nack)