Add support for posting comments

[reavessm]

Probably should merge this after #5

[msuchane]

Thanks for the PR, @reavessm. I've finally reviewed it and made some changes:

• This crate already uses log for logging so I replaced tracing with log. If you'd like to reintroduce tracing, though, I don't mind -- I just think it should be consistent across the whole crate. We could have both side by side.
• I've also changed the logging level from info to debug because I believe that's more appropriate for this kind of information.
• I saw a TODO item about commenting as myself or as another user. I've added an implementation for specifying another user but I can't test it now. Could you please try it out and let me know if it works for you?