Basic Interfaces of ShoppingCartService and related classes

Adjust:
- ShoppingCart
- ShoppingCartPosition

Rename:
- ShoppinCartService to ShoppingCartService

Add:
- Visitor
- Visitable
Merge branch 'master' of https://github.com/Thagor/shop-api

Conflicts:
	.classpath
	src/main/java/de/is/project/shop/api/domain/Product.java