Add API docs for commands API

[AlMcKinlay]

Forgot to add this when I added the functionality a few months ago.

[AlMcKinlay]

Ah no, there's a mistake in this.

[AlMcKinlay]

Ok, I have removed the info about the runAsUser command on the Commands API because I realised it doesn't even work right now, and even if it did, it's kinda pointless because you need to pass through the client, which if you had you could just call runAsUser on the client itself.

So I'm going to fix that in another thing and then put the docs up for it.

[xPaw]

Can you add explanation for what the callback arguments are? function(client, target, command, args)

[AlMcKinlay]

Done. I also added docs for the client object as a separate page, because:

1. It'll get too messy if that's in the same page
2. I'm sure we'll use client elsewhere as well

[xPaw]

I merged #220, can you review this PR please.

[AlMcKinlay]

Most of this was included in #220, but there was 1 error that I've fixed for this PR.