update README, remove unused dependencies, remove broccoli files

This is a follow-up to #1126. It updates the build process to use [gobble](https://github.com/gobblejs/gobble), which is a new build tool that came out of my day job. It's still something of a work-in-progress, but has reached a point where it's stable and fast enough to start using for real.

I've also updated the README to give more useful and up-to-date information for would-be contributors to get started.