Don't concat scripts for debug builds

Hello,

From the beginning of EAK, scripts have been concatenated together for all builds. Later, [grunt-concat-sourcemap](https://github.com/kozy4324/grunt-concat-sourcemap) was [added](https://github.com/stefanpenner/ember-app-kit/commit/580525dfc5) to improve debugging.

This PR simplifies the build process by using [grunt-html-build](https://github.com/spatools/grunt-html-build) to inject sets of link and script tag, as well as HTML, into `app/index.html`. By doing so, it also removes the need for `grunt-preprocess`.

The link and script tags are injected inside of `grunt-usemin` markers so that they are concatenated and minified during a `dist` build.

If there are any questions or problems with this PR, let me know. :)

Cheers,
Josh