Provide external logger

[bijoor]

Would like a way for roboto to use the logger provided by the contained application.

Possible approach (not tested):

1. Add an option customLogger (or something like that)
2. In constructor add this._log = this.option.customLogger || log;
3. Replace all log.* with this._log.*
4. Add a parameter to itemLogger constructor to accept external log object itemLogger(this._log)

Thanks for some awesome code BTW!