refactored and added @Pattern annotation

need to disable one test, that fails the build

http://fisheye.jtalks.org/changelog/JTalks-common/jtalks-common-model/src/test/java/org/jtalks/common/mixin/ModificationAwareImplTest.java

here is the change:

@Test
@Test(enabled = false)

The following constraints should be apllied on User entity:

User
@NotNull
@Length(min = 2, max = 25)
username
@Email - (our custom email annotation)
email
@NotBlank
@Length(min = 4, max = 20)
password

Regarding password and username - is it possible to check in phpBB what are the constraints? Or these are those constraints? Just to ensure that we have same or broader ones.
Anyway, the constraints for user are not relevant for Poulpe, so we approve whatever Larks say.