Introduces PersistenceRealm#getAccountByUsername method.

It is introduces in order to refactor the code to separate getting
account in the separated method. 

The method returns the AccountEntity with the same username and throws
UnknownAccountException if the account has not found, or if its password
or its password's salt is null.

Signed-off-by: Mario Krizmanic <mario.krizmanic@gmail.com>