Add support for QoS 2

[njh]

Support for local persistence for a message queue is a requirement before QoS 2 can be implemented.

[njh]

@eLocker-UK it would be useful to know why you require QoS 2 and what you use case is.
One of the things I have struggled with a bit is working out an API for providing feedback when a publish fails.

[magynhard]

We use ruby-mqtt for test automation and we have a use case with QoS 2 to test.
Unfortunately we can not, as it is not yet supported by ruby-mqtt.

When trying to use QoS anyway, i get this error:
Connection error on publish: execution expired

Hope support for QoS2 is coming soon.

[ara-mark]

Hi! Is there any chance for qos 2 support in the future? thanks!

[zernel]

I encountered this issue with version 0.5.0, but it was resolved after upgrading to 0.6.0. It seems the problem has been fixed.