Changes

Adds user registration service and tests
This commit adds the user registration service with email verification. 
For this, functionalities preRegistration and confirmRegistration have 
been added to UserEJB. In addition, Mail, UuidGenerator and 
ConfirmationMail classes and MailSender and TokenGenerator interfaces 
have been created.
For testing this functionalities, TokenGeneratorStub and EmailSenderSpy 
classes has been created.
Besides, the property "accepted" of User entity has been deleted because 
it is redundant and the VerificationToken entity has been modified for 
correct operation.
The name of packages have been unified.
Some documentation has been added.