Service: add equals and hash code method for comparison
Service DAO: Interface definition and in memory placeholder
Service DAO: basic in memory implementation & tests
Customer DAO: basic in memory implementation & tests
Employee DAO: in memory implementation; Inheritance for Employee and Customer
Appointment DAO: simple implementatino + unit tests
Merge pull request #4 from Fabian-K/daoImplementation

DAO Implementation for Appointment, Customer, Employee and Service
EmployeeService uses InMemoryDAO functionality