Logging integration for 3rd party libraries that directly use Go logging

[benhalstead]

The behaviour for 3rd party code that directly uses Go's https://golang.org/pkg/log/ logging framework running within a Granitic application is currently undefined.

This issue is to:

1. Explore and document the current behaviour (in the reference manual)
2. Propose a solution for managing making sure that this sort of code plays nicely with Granitic's logging framework.

Ideally we'd like to be able to intercept the logging calls of the 3rd party code and control the output using standard Granitic concepts (components, log levels etc)