Changed folder name "Database" to "Model", and add DbHelper.java, Pagination.java, CusService.java into the folder

DbHelper class mainly used to provides database connection method and
some low-level functions for the pagination

Pagination class is used to store and encapsulate page properties

CusService class is used to integrate all those functions and provides
T-SQL sentences for the program, so, in view, this class is the only
class could be called. Maybe later, the other main functions like add,
remove, and edit also should be integrated in this class.