Add ngdocs support to Grunt

I noticed you had `@ngdoc` statements in your codebase, but ngdocs wasn't in your Gruntfile. I added ngdocs to the Gruntfile and fixed a syntax error in one of the `@ngdoc` statements. 

To create documentation, simply run `grunt ngdocs`. This will populate the docs directory with all your doc pages (assuming you have already run `npm install`).