Add lint script and rules

Add a `lint` npm script; there's a .jshint file here, but it's not being used. It may be worth looking into a strict eslint rule-set that makes sense for LTS versions of NodeJS (4.x, 6.x).

Some options may be:
https://github.com/thenativeweb/eslint-config-es
https://www.npmjs.com/package/eslint-config-airbnb

etc..

The airbnb config, while great, aggressively suggests bleeding edge features that need transpilation support from babel, so it may not be a great fit here.

Start by inspecting the existing .jshint file and the repository's npm configuration. Compare the suggested ESLint configurations against the stated Node.js 4.x and 6.x compatibility needs. Done means an npm lint script uses an agreed rule set without requiring unsupported language features.

Written by the indexing model from the issue text.