Support for transactions

Sequelize (http://sequelizejs.com/docs/latest/transactions) transaction implementation is achieved by establishing a new connection to the database.

PDO has transactions. However, leaving transaction handling to the user at the PDO level would create a direct dependency, not documented in MOA documentation. Sequeliser implementation uses a callback. MOA could implement similar approach to contain transactions. Existing implementation following similar pattern is Laravel, http://laravel.com/docs/database#database-transactions.