Variant images

Introducing variant images in Timber.

[Demo](http://timber-demo.myshopify.com/products/bjml-mystic-water-clean?variant=780875897)
- Size 26 has no variant image and is sold out, to test the variations

#### Cart
- Use `item` rather than `item.product`

#### shop.js
- Added `timber.switchImage` to be used when a thumbnail image is clicked and the variant is switched (`product.liquid` line 195)

#### product.liquid
- `featured_image` line 26
- `variant` line 60
- Remove check if an image exists (just caused duplicate code)
- Hide quantity selector if product is sold out or unavailable

cc/ @christopherlobay  @maartenvg @celsodantas @carolineschnapp