Fixed issue #3: prevent to create a user with a null or empty ID
Added functionality to issue #3
Added control over invalid chars in userID.
Added static vars User::minUserIDNChars and User::maxUserIDNChars to
limit userID length
Renamed exception userIDEmptyNull to userIDInvalid
Changed classes User, UserManagment
Added respectives tests
Clean up in isValidUserId prep_replace
Merge pull request #9 from paulocmguerreiro/master

Fixed issue #3: prevent to create a user with a null or empty ID