Added object filters for MongoDB ODM like ORM does.

It is very useful to have object as a filter.

I have made the changes but I need some review on the code or maybe more things need to be done (probably some test)

**BTW**, I realize that in the Configuration.php there are
```
// Doctrine\ODM\MongoDB\Configuration
$this->attributes
// and
$this->_attributes
```

It confuse me when I edit the config file, so I wonder it is some mistake on this, in ORM it uses `$this->_attributes`

*Filter object In ORM* : https://github.com/doctrine/doctrine2/pull/434/files