Tilesets API

[critical-path]

This attempts to address #152. I would be happy to make changes to the code, tests, etc. Thank you!

[sgillies]

@critical-path there are a few conflicts to resolve after the merger of #235. My schedule is full for the rest of this week, but I think I'll be able to review this next week.

[critical-path]

@sgillies

I have resolved the merge conflict. Please let me know if you would like additional changes. Thank you!

[sgillies]

Thanks @critical-path ! Tilesets API is the most crucial of these PRs to review, I think, unless you have other priorities.

[critical-path]

@sgillies

Thank you for the opportunity to contribute to this project! My apologies if I tagged you too many times - I am still learning GitHub workflows and conventions.

I defer to you on prioritization. It seems very reasonable to review Tilesets first. Thank you!