WIP for a doctrine compiler pass to handle mappings

The idea is to show how the doctrine bundle could help bundles register paths (for xml mappings) for their model classes, instead of copy-pasting models into Entity and Document folders like FOSUserBundle currently does.

**Note that the compiler pass and handling belongs into the DoctrineBundle and not here. This is just for simplicity to demo the idea.**

All a bundle has to do when providing models that can be used with several storage layers is to implement `getDoctrineOrmPrefixes` (and respective methods for mongo, couch and phpcr). as in 

    public function getDoctrineOrmPrefixes()
    {
        return array(
            realpath(__DIR__.'/Resources/config/doctrine/model') => 'FOS\UserBundle\Model',
        );
    }

To run this code, set up a clean symfony 2.2 and load the user bundle as `FOS\UserBundle\FOSUserBundle($this)` to pass it the kernel instance.

## Questions:

### is this sane?
### interface?
how can we make this happen? the interfaces would be in DoctrineBundle or doctrine bridge, but that would mean any bundle implementing will depend on a version of doctrine that provides this interface, so no elegant BC. we could provide a (transitional?) bundle that just provides the compiler pass and bundles could depend on that. that bundle should be compatible with doctrine 2.3 and 2.4.
the alternative probably would be duck-typing - but then the bundle would still need to provide mappings for the Entity / Document class as before, and it might lead to unexpected behaviour as no mapping for the model classes is found with a too old doctrine bundle.

## Alternatives

### Tagged services
Bundles could provide a service and tag it with some tag. This is a lot of overhead as each bundle would need to implement its class. And we still should duck-type as interfaces would still be problematic.

### Prepend Configuration
Maybe we could leverage this new feature to have bundles prepend mappings to the DoctrineBundle
http://symfony.com/doc/current/components/dependency_injection/compilation.html#prepending-configuration-passed-to-the-extension
Not sure if that would be very elegant or very complicated.