uuid fields for xml/yml driver

again without that typo and with one commit only.
(See https://github.com/doctrine/phpcr-odm/pull/437) for all commits.

What did i changed?
For xml/yml driver didn't exist a mapping for the uuid. Just for the annotation driver. So i created both. I added the condition for both drivers with creating the mapping the `ClassMetadata` needs. 
Created a new test with a fixtures too.