Add callbacks for self JOIN/PART messages

[zneix]

Not sure if it's good to invoke the client callbacks within handle functions for JOIN/PART but it just seemed the simplest.
I'm not familiar with library's code much, thus I don't have a good idea about what potentially relevant tests I could add, but I'm open to experimenting and writing something. thanks pajlada for help figuring that out

One small side-note I have: name of the callback OnUserJoinMessage might be confusing, indicating it's carrying JOIN message for our user because of the User part in its name (ideally OnJoinMessage and OnSelfJoinMessage would make most sense to me). I know that's the name that was there from the beginning but I think it's only confusing if anything.
That's nothing that I can change though, just pointing that out...

Closes #178

[coveralls]

Coverage increased (+0.02%) to 98.019% when pulling f773c9a on zneix:zneix/feat/handle-self-joins-parts into fde60d2 on gempir:master.

[gempir]

Yeah I see how that could be a little misleading. But I think it's not worth breaking API over. Could you also update the readme to document the new callbacks?