Add some support for comments

[bentranter]

Adds some more support for API methods -- this one for adding comments. I'm still trying to figure out the most efficient way to test all these methods, and I'll submit a PR to add full test coverage once I've got that figured out.

[dinever]

Hi @bentranter,

Thanks for the contribution, looks really good. Could you please update this PR with latest changes from master so that I can merge it?

[bentranter]

@dinever just updated, so if you think it's all good then we can merge it

[dinever]

Looks good, thanks @bentranter!