Update Gruntfile to run on Grunt 0.4, minor change to doc image links

Rather than use grunt 0.3 locally to generate the docs, I updated `grunt.js` (now `Gruntfile.js`) to use Grunt 0.4. Larger changes:

* [the jshint contrib plugin](https://github.com/gruntjs/grunt-contrib-jshint) has different options than before, so some changes may be needed based on your preference. 
* The watch task is removed (it was never registered anyway)