Changes

tsk31 Adds the functionality for login and logout
Adds the controller where the necessary functions for login and logout 
are implemented. Also added checks for valid login credentials
Adds the UserService file where the user in session is obtained

tsk31 Adds the tests to check the login functionality
Adds IsEqualsToUser and UserServiceTest classes that implement the
necessary methods to perform tests on the Login
Adds in UsersDataset the necessary methods to check if the user with
which the session is started is an existing user