Log inside library

[Rafael Dantas Justo]

Inject a logger into the library to add informational and debug level messages. This is useful when debugging a problem to understand what is going on.

For now we could use the following log interface:

type Log interface {
	Debug(args ...interface{})
	Debugf(format string, args ...interface{})
	Info(args ...interface{})
	Infof(format string, args ...interface{})
}