Added a way to enable auto_mapping option using multipe entity managers

Finally, I fond some time to create this PR. This should close https://github.com/doctrine/DoctrineBundle/issues/60

I've also added  a small test. Feedbacks are welcome!

(if merged, this will require some lines on SF documentation...)
I'd completely reverse the order of your foreach here. I'd loop through definedBundles and find an EM that is the candidate for mapping, not an O(n^3) solution. I assume that multi-auto_mapping should be verified somewhere else, not when deciding the bundles mapping.

It would turn your algorithm into something like this:

```php
private function validateORMAutoMapping(array $entityManagerConfigs)
{
    $emAutoMapping = null;

    foreach ($entityManagerConfigs as $name => $entityManager) {
        if ( ! (isset($entityManager['auto_mapping']) && $entityManager['auto_mapping'])) {
            continue;
        }

        if ($emAutoMapping && $emAutoMapping !== $name) {
            throw new \LogicException(sprintf('You cannot enable "auto_mapping" on more than one entity manager at the same time (found in "%s" and %s").', $emAutoMapping, $name));
        }

        $emAutoMapping = $name;
    }
}

private function collectAutoMappings(array $entityManagerConfigs, array $bundles)
{
    foreach ($bundles as $bundle) {
        $emCandidate = null;

        foreach ($entityManagerConfigs as $name => &$entityManager) {
            // Specific bundle mapping
            if (isset($entityManager['mappings'][$bundle])) {
                $emCandidate = $entityManager;

                break;
            }

            // Auto-mapping
            if (isset($entityManager['auto_mapping']) && $entityManager['auto_mapping']) {
                $emCandidate = $entityManager;
            }
        }

        // If we could resolve at least one EM
        if ($emCandidate) {
            $emCandidate['mappings'][$bundle]['mapping'] = true;
        }
    }
}
```