Fixed Mongo_Collection::remove_safe

The main idea is that the `$criteria` param may be empty, in this case Mongo should remove all documents from collection.

Also I have made more strict list of accepted arguments, and amended PHPDoc docblock.

+ http://docs.mongodb.org/manual/reference/method/db.collection.remove/
+ http://www.php.net/manual/en/mongocollection.remove.php