NEW: Password Hashing and user verification

* Created class PasswordHash which provides funcionality to hash passwords
and to compare hashed passwords
* Added Library Apache codec needed for class PasswordHash (in case of
problems just click on project -> properties -> libraries and add folder
called Apache-commons-codec-1.9
* Modified class User
* Added 3 new methods (addUser, getUser, verifyUser) into class ParkingLot