Add logging support

As soon as `sqlalchemy` logging is enabled

    import logging
    logging.basicConfig()
    logging.getLogger('sqlalchemy.engine').setLevel(logging.INFO)

`alchimia` cannot execute any statement:

    exceptions.AttributeError: 'TwistedEngine' object has no attribute 'logger'

This PR adds logger to `TwistedEngine` in order to use sqlalchemy logging routines.

These attributes should be provided by proxying the underlying `self._engine`.