Refactor using a combined QosPid struct.

[vincentdephily]

Related to bug #6, but deserves to be treated separately as it is an API change instead of a bugfix:

In my own code I've found it useful to have both a

pub enum Qos {
    AtMostOnce,
    AtLeastOnce,
    ExactlyOnce,
}

and a

pub enum QosPid {
    AtMostOnce,
    AtLeastOnce(Pid),
    ExactlyOnce(Pid),
}

The QosPid struct would have made bug #6 more obvious, and would prevent similar bugs in user code (where it's easy to not have qos and pid in sync).

I suggest to do the same in this lib, replace the two fields qos:QoS, pid:Option<PacketIdentifier> with a single qospid: QosPid as defined above. This goes towards the "hard to misuse" goal of good APi design (and makes the struct smaller as a bonus).

Like #10, this is a breaking change. Hopefully we can lump a few together.

[vincentdephily]

Probably want to wait until the branch is merged before closing ;)