cleanup id strategy logic and validation step

this involves a small BC break because if the parent is mapped but no nodename, we now prefer the (recently added) autoname strategy over the assigned id. the id priority is now:

* explicit strategy defined on the id mapping
* parent strategy if parent and nodename are both mapped
* autoname strategy if the parent is mapped
* assigned id strategy if the id is mapped
* in any other situations, things can not work and an exception is thrown

we might want to update the documentation with this as well. if it al makes sense, i can do that.

fix [PHPCR-60](http://www.doctrine-project.org/jira/browse/PHPCR-60)