replace deprecated code to use new PHP Mongo API

This pull request will resolv these kind of issues :

    ERROR: ErrorException [ 500 ]: Mongo_Connection::__construct(): The Mongo class is deprecated, please use the MongoClient class ~ /path/to/kohana-mongodb/classes/Mongo/Connection.php [ 34 ] in /path/to/kohana-mongodb/classes/Mongo/Connection.php:34
    ERROR: ErrorException [ 500 ]: MongoCollection::save(): The 'safe' option is deprecated, please use 'w' instead ~ /path/to/kohana-mongodb/classes/Model/Mongo.php [ 112 ]

I'm using the mongodb php module from here :
https://github.com/mongodb/mongo-php-driver