Refactor to move to OO and stuff...

First round of refactoring of app.

- Created class `MBC_TransactionalEmail`
- Setup __construct() to pass in MessageBroker object (needed for sendAck in callback method)
- Added StatHat support and several reporting points for various transactions supported by the app.
- Created `buildMessage()` method, needs further refactoring in round two. See:
https://github.com/DoSomething/dosomething/issues/2051