Add binstubs, bin/setup, CONTRIBUTING.md

Improve developer experience of getting involved.
Use `hk` instead of `heroku`
Remove `tail` subcommand

The original reason for the `tail` subcommand was that the `heroku`
interface of `logs --tail` was cumbersome. The new subcommand in `hk` is
good and we shouldn't add to the surface area of the API.

    staging log
    production log