Extra lazy get for inverse side of many-to-many

This is the cange requested by @stof and @beberlei in PR #710. It implements an extra lazy get on the working side of the relationship. As mentioned in #710 the unit tests fails for reasons I don't quite understand. The error I get is:

```
1) Doctrine\Tests\ORM\Functional\ExtraLazyCollectionTest::testGetIndexByManyToMany
Exception: [PHPUnit_Framework_Error_Notice] Undefined index: joinColumns

Trace:
/home/sander/src/doctrine2/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php:1665
/home/sander/src/doctrine2/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php:1610
/home/sander/src/doctrine2/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php:1701
/home/sander/src/doctrine2/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php:1115
/home/sander/src/doctrine2/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php:746
/home/sander/src/doctrine2/lib/Doctrine/ORM/Persisters/ManyToManyPersister.php:55
/home/sander/src/doctrine2/lib/Doctrine/ORM/PersistentCollection.php:529
/home/sander/src/doctrine2/tests/Doctrine/Tests/ORM/Functional/ExtraLazyCollectionTest.php:578
```

If someone with a little more understanding of the Doctrine internals can help me fix this, then we could restore some of the functionality that PR #710 had to remove.