Use jshint instead of gjslint

Add jshint configuration, configure npm to run jshint in "pretest".

Fix existing issues to make jshint pass.

@3y3 please review

The most annoying change is replacing checks like `to.be.false` with `to.equal(false)`. Perhaps we could use [must](https://github.com/moll/js-must) instead of chai. Do you have any opinion on that?