MongoDB support

Need support for
- transactions
- record helpers (haveRecord)

looks like we have to effectively copy paste the entire bunch of record helpers and swap the active record refence to mimic. and for transaction, both connection watcher and transaction forcer is hard refenced to yii/db/Connection, to have transaction support we have to clone them and swap references to yii/mongodb/Connection and yii/mongodb/Transaction.

@samdark - Ideally, can we have a common parent for both yii/db/Connection and yii/mongodb/Connection? something like yii/db/BaseConnection.

Thanks