Add database/sql Db.Prepare & Transaction.Prepare equivalents

Our application requires access to the `Prepare` function as defined by database/sql. Up until recently, it wasn't needed in transactions, so we got access to it via DbMap.Db - not ideal, since it breaks gorp's encapsulation from the app's point of view.

This exposed Prepare on DbMap & Transaction; it's just a pass through to database/sql's functionality.