uuid fields for xml/yml driver, test for all uuid stuff

created item in xsd file

add identifier to test model

added fieldName key to mapping for xml driver

simplify part for yml mapping

added id's to xml/yml tests

Update doctrine-phpcr-odm-mapping.xsd

Update AbstractMappingDriverTest.php

deleted useless todo
Merge pull request #438 from ElectricMaxxx/uuid-driver-fix

uuid fields for xml/yml driver