Add jsvalidate to grunt build process

This adds the [jsvalidate](https://github.com/ariya/grunt-jsvalidate) package to our build process.  It's fast, so I set it to run by default.  It'll help catch any syntax errors in `crafty.js`.

I tried having it run on all the source files individually, but it was a bit kludgy.  ( `core.js` wouldn't be valid by itself.)  Maybe someone else can have better luck with that.