Add accounts tokens endpoints (work by @morgant)

[samfader]

@morgant Per #43 (comment), putting your PR in here with a local branch to ensure tests pass.

[samfader]

@morgant Thoughts on these failing tests? I don't think they are related to a missing token; that appears to be correctly being pulled as an environment variable.

I haven't looked deeply due to time constraints. Let me know if you think it's something on the admin side.

[morgant]

@samfader Thanks for your continued work on this! Looking at the Travis CI job log, I don't see the new MapboxUsername environment variable being set, only the MapboxAccessToken. Please correct me if I'm wrong. Is that something you have access to set? If so, it should be the Mapbox username associated with the account the access token was created under.

[samfader]

Hey @morgant, thanks for your patience. I've been meaning to come back to this for a while. I've decided that the best approach will be to mock these tests - I don't feel great about creating and deleting tokens on a real account. I'm looking into this approach now.

[woodhull]

Just re-upping this as a feature request, it's been "almost mergeable" as a feature across two different PRs for a few years now.