Get and create images for a specific product

Added 2 methods to get and create images for a specific product. 
```
GET /products/id/images.json
POST /products/id/images.json
```
The problem is the naming of the method "get_product_images" and "create_product_images" aren't quite right because all product resources are named plural. But I don't really know how to name them.

This should be added for every product specific resource like videos or discount rules.