add Sale and Customer classes
start work on ModelFacade
add constructors taking interface objects
add Id int and Deleted bit
add Model and Data facades

Implementations not finished
add Entity for Id, Deleted and LastUpdated

ModelControllers can inherit from abstract controller
Will be used for all non-stupid objects (meaning not Enums)
add CustomerController
add SaleController
add creation of lists in controllers
edit ModelFacade's Create method

CreateCustomer and CreateSale now takes no arguments.
This way, UI can call modelFacade.CreateCustomer and the UI gets an
interface back to work with.
Merge pull request #6 from TBroedsgaard/StartSaleModel

Start sale model