Added comment to the Users class, why the class has the name Users and
not User. Added comment to the isExpired method and annotation to the
toString method.

Change-Id: I6400e880f3790b9d5fcdac8879d88300b3c9e932
Signed-off-by: Peter Merkert <p.merkert@sap.com>
The AccountServiceTest now works with the correct service parameter
names.

Change-Id: Ic6c18639c1639f7041f10c01bdf5ef13264c33a8
Signed-off-by: Peter Merkert <p.merkert@sap.com>