Grunt tasks

This is a contrast to #256, which uses concurrent console tasks to accomplish file concatenation and Shopify's theme gem activity.

This method uses `grunt-shopify`, as purposed in another discussion. 

- All styles are separated into a `stylesheets/` folder
- `stylesheets/` folder is watched and concatenates to `assets/timber.scss.liquid`
- Run `grunt` to concatenate CSS, optimize images, and upload to your store
- Run `grunt zip` to generate a Shopify-friendly zip file with only the required files.

Kudos to @graygilmore for getting the jump on this implementation.

cc/ @stevebosworth @Shopify/fed @fredryk