Mongodb "$in" queries not working

Mongo [$in queries](http://docs.mongodb.org/manual/reference/operator/in/) are not working anymore, failing inside `libs.utils.replace_keys` complaining that `unicode` does not have an `iteritems()`.

I must admit that the dict comprehension is a bit difficult to understand and *I might have missed it*, so please, read the patch carefully.

Though, I could fix the problem and `$in` queries are providing me expected results while passing tests.