add eslint to 'make check'

[trentm]

Linting is a good thing. Bunyan should be doing that. I've been using javascriptlint in a lot of work projects, but a friend has had good luck with eslint. Let's try that. If that is working well, we might just replace current jsstyle usage with that.

• get starter eslint config from josh and a 'make check-eslint' in place
• separate lint and style config files and check those separately: make check-eslint-lint and make check-eslint-style
• refine the config files to a style I like (hopefully quite similar to what josh has already
• hook it in to make check