Support other loggers

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.

```golang
// in DbMap
logger interface {
	Printf(format string, v ...interface{}) 
}
// vs.
logger    *log.Logger
```

Happy to supply a PR if there's interest...