JSLint with Grunt

The Development page on the Wiki asks developers to follow JSLint, and now a runnable task to do just that can be part of the project!

* `npm install` will download `grunt-contrib-jslint` and `grunt-contrib-watch`
* `grunt jslint` lints `Gruntfile.js` and all of the .js files in `src/ext/`
* no more duplicating JSLint directives in comments at the top of files; all the configuration is set in `Gruntfile.js`
* `grunt watch:jslint` will start a process to watch for file changes and automatically run `grunt jslint`