add logging levels to log() function

[landlessness]

would be nice if Zetta had log levels to help with grep on output

TRACE < DEBUG < INFO < WARN < ERROR < FATAL

[mdobson]

@AdamMagaluk did you have thoughts on this? I know you wanted to use like winston or something? I'd like to implement this, because it'd be nice to have. Also, we have a few other issues in the logging category that may be good to knock out.

[AdamMagaluk]

I didn't want to use winston in our logging but I did want to make it easier for people to plug in winston or bunyan. Hence having logging levels that match up to winston/bunyan would make it easier.

[AdamMagaluk]

See #113

Supports info, warn, and error levels.