Merge support

it basically works. the tests are not yet sufficient and there are some cases not yet covered. also there is some redundant code that needs to be refactored.

open questions:
1) should he have special handling to prevent overwriting child properties with null?
2) should we have special handling to prevent overwriting node/nodename/version name/version created/locale with null?


http://doctrine-project.org/jira/browse/PHPCR-13