js: Update dev env instructions
js: Update test deps
js: Pass grunt-browserify errors on
js: Update dev dependencies
js: Upgrade to when.js 3
js: Simplify _send() rejections
js: Add TODOs for rejecting promises with Error

This is recommended to get proper stack traces, according to
https://github.com/cujojs/when/blob/master/docs/api.md#a-note-on-javascript-errors
js: Reject promises using Error objects

This is recommended to get proper stack traces, according to
https://github.com/cujojs/when/blob/master/docs/api.md#a-note-on-javascript-errors.

The new Error objects has the exact same properties as the objects we used to
reject promises previously, thus this should be fully backwards compatible, but
improve debuggability.
js: Add ServerError and ConnectionError types
js: Fix unhandled promise rejection in test
js: Connect to /mopidy/ws without trailing slash

This is the recommended URL since the switch to Tornado as web server.
docs: Document Mopidy.js settings (fixes #702)
docs: Improve Mopidy.js Promise usage examples
Merge pull request #744 from jodal/feature/js-updates

Various updates to Mopidy.js