Reorganize repo

- Revamped gruntfile with `compile` and `dist` tasks replaced with `build` and `ship`.
- Everything moved into src directory.
- `grunt-usemin` is used in `_layouts/base.html` to reference minified versions of assets for production.
- There are now separate `watch:js` and `watch:css` tasks.
- Run `sheer serve` from the `dist` dir instead of the project's root.

@ascott1  `npm install` and `grunt` and let me know if you run into any problems.