Address #2 - Capture metadata as transaction attributes

This pull request adds the following functionality:
 - Allow NewRelic to capture winston metadata as transaction attributes
 - Added istanbul coverage - 100%
 - Handle metadata containing objects/functions

![transaction attributes](https://cloud.githubusercontent.com/assets/1964119/3249120/82579420-f195-11e3-9300-b8d90ea5f6e7.jpg)