Update comment

Hi,

I added a simple request to update an existing comment - the one feature I couldn't find in your incredibly useful library.

I'm pretty sure I screwed up the architecture by adding a new Cards.UpdateComment next to Cards.AddComment (because that's under /actions/ endpoint, not under /cards/), but I couldn't find a better place. Also, I was missing an interface for comment actions, but I didn't want to change too much of your code.

Maybe you can use my few lines. Please let me know if I can do anything else to help or make my few lines fit better into your library. Thanks a lot for your work!