Fixed Mongo_Collection::remove_safe

[sergeyklay]

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