Add create product type link to create product dropdown menu
Category caching; index all level of categories for products in lucene so that we can search products by their parent categories
Search: returns only the top most level categories in facet result
Add final price range info in Product api model
Multilingual: caching for languages, brands and categories
Refactor customer management
Remove confusing Customer.AccountId, the customer account connection between commerce and cms now use email instead
Orders should not be deleted, and remove order creation function in backend cos it's buggy and orders should almost come from frontend
Remove vitaminstore sample because we have a better and more generic sample
Refactor brand, category management and refine models
Remove all interfaces for services like IBrandService, ICategoryService etc

- These interfaces is over abstracted, we actually don't want developers to replace service implementations but extend domain logic by activities or some more narrowed interfaces
- These interfaces do not expose commerce instance information,
  so when we want to manipulate data for an instance, we have to build a instance context, this is not convenient, so remove it for a easier life
Minor refactoring on repository