[GH-479] [WIP][Mapping] Ported some of the yaml driver to use Symfony config

This issue is created automatically through a Github pull request on behalf of kimhemsoe:

Url: https://github.com/doctrine/doctrine2/pull/479

Message:

Looking for some input. How much validation and normailization should i push to the configuration ? Should i use default values so we can remove alot of lines from the driver ?

Is the way im allowing to extend the configuration good enough for Gedmo and others (untested)?