Add username from CONNECT to client after authenticate succes

[faceless2]

Re issue #147, the basic requirement in any authentication scheme is knowing the username specified during the CONNECT phase. So I think this really should be in the core API, rather than added by users.

This patch adds a that as the Client.username property.

[robertsLando]

You should add a unit test that covers this

[robertsLando]

I would also know @getlarge and @mcollina thoughts on this

[getlarge]

It makes sense to have the username by default in the client, personally i always add it manually :)
And yes some tests would be needed and the types should be updated too.