Created an endpoint for the root of REST API.

Notice that each API URL pattern is named.
Added pagination.
Introduced `HyperlinkedModelSerializer`.
Refactor product index page.

1. Since the related name `images` is used in the `gem_store.models.Image` to show the reverse relation between `Product` and `Image` models, every `image_set` is replaced by `images`.

2. Fetching the first image of each product using *dot lookup* syntax in Django template language. Ex. `product.images.all.0.image.url`.