Upgrade to work with grunt 4, fixes #4

This should be a good start to making this work with grunt 4.  I've spent some time testing it out and making sure the linting and watch tasks still work.

I had a problem with the grunt-contrib-jshint task failing because of a recent syntax change from `this.file.src` to `this.filesSrc` (I have a PR in to [fall back gracefully](https://github.com/gruntjs/grunt-contrib-jshint/pull/15), but they recommend making sure the dependency is at least `0.4.0rc7` instead).  I've updated the reference in the package.json, but in case you see an error related to 'cannot call method forEach` that is probably the culprit.

I added the `"use strict";` on the lint targets because it was complaining and I wasn't sure if you wanted me to disable that in the jshint options.

I'd also probably consider this at least a minor npm version bump so people running grunt 3 aren't caught off guard, but I'm definitely not trying to tell you how to handle your biz.