Using theme gem instead of grunt-shopify

Remove `grunt-shopify` in favour of Shopify's `theme gem`.

No reliance on a third-party plugin. Can use `theme gem` natively if you want without Grunt at all.

- `grunt` runs `theme watch` alongside CSS concat and image optimization
- 'grunt deploy` runs tasks then uploads all files to store
  - Use `ignore_files` in theme gem's config file to determine what is uploaded

cc/ @stevebosworth @mpiotrowicz @tylerball @stevenmichaelthomas