Remove formatting functions

Incompatible changes:

• Remove all formatting functions: Debugf, Infof, Warnf, Errorf because they do not fit well with structured logging f5e3c49
• Name all embedded fields in adapters bd39ae9

Bugfix:

• Fix zap field logging dadca38

Add WARN level

• Add new WARN level.
• Improve glog adapter to log error if given
• Fix invalid SkippedCallerFrames sent by formatting logging functions such as logger.Infof, logger.Debugf and logger.Errorf

Add documentation to public API

v0.1.1

Document API and adapters

Initial release

This release is ready to use API with 7 available adapters:

• logrus
• zap
• printer (standard packages: fmt or log)
• glog
• log15
• zerolog
• context

The API is not yet final though, and could be changed in the near future (although it should not change dramatically).