Support other loggers

[mwielbut]

It would be great if the logger on DbMap was defined as an interface with the single Printf function needed rather than the full std log.Logger struct to make it easy to supply custom loggers ie. Logrus.

// in DbMap
logger interface {
	Printf(format string, v ...interface{}) 
}
// vs.
logger    *log.Logger

Happy to supply a PR if there's interest...