Refactor code by Gary

[GaryLFF]

1.Fix the variables and function naming standrad.
2.Fix the input validation in user input, like DueDate must be more than datatime now and need a datetime valided. Toy count must be a positive integer.
3.In order to make the business isolation, we need to make the code into different layer accoridng to the business logic.
4.Make the Report more genreal, we need to create a base class, and using templete class to decide which report shoud be created.
5.Main method shoud not has much logic, follow the Single responsibility principle.
6.Remove no using code and change the class's access modifier.
7.Fix Improper variable type, such as using decimal in Price rather than int.
8.Create the Domain layer to define the business model, and Serive layer to handle complex business logic.
9.Shoud have Unit test(TBD)