Update logging API to support named loggers

Fixes #7

- Updated logging API to support named loggers
- Added logger implementations for NLog and log4net
- NuGet build targets for logging integration
- Updated samples for new logging API
- Up-revved to version 0.1.12.8 (have to specify a Chinchilla version in dependencies section of new NuGet logging integration packages)

Note that you can set the logger factory to be used like:
`Logger.Factory = new NLogLoggerFactory();`