Make logger interface based with minimal methods

[jeevatkm]

The goal is to make the Resty logger interface based in v2 with minimal methods that Resty user could choose their choice of the logger.

Also if possible add default implementation using log.Logger compliant with the new interface.

[jeevatkm]

Changes:

• Client.SetLogger accepts resty.Logger interface

// Logger interface is to abstract the logging from Resty. Gives control to
// the Resty users, choice of the logger.
type Logger interface {
	Errorf(format string, v ...interface{})
	Warnf(format string, v ...interface{})
	Debugf(format string, v ...interface{})
}

[jeevatkm]

Its done 😄