Grunt-preprocess, windows compatibility, changes to grunt pipeline

- `index.html` is now in `app/index.html`
- `index.html` is now shared between tests and app. I think some users where waiting for that for quite a bit.
-  The `build:dist` task is now just called `dist`. This makes sense because `build:debug` is useless by itself. Thus `build:dist` and `build:debug` are now just used internally and the `dist` dist task is now the public one.
- `grunt-preprocess` now replaces `dum-munger`. This fixes some issues on windows. https://github.com/stefanpenner/ember-app-kit/issues/247
- Loom is now optional. Added install instructions and description in Gruntfile.js. https://github.com/stefanpenner/ember-app-kit/issues/247
- Fix: The development server didn't serve the test directory properly. https://github.com/stefanpenner/ember-app-kit/issues/251