Add and use Logger / FancyLogger interface

[mrbanzai]

Permit using loggers other than stdlib, eg. golog, logrus, etc.

Please ensure the following is present in your Pull Request:

• Unit tests for your change
• Integration tests (if applicable)

Pull Requests that are small and limited in scope are most welcome.

Thank you!

[ghost]

@mrbanzai Thanks for the PR. I've pinged a golang guru to ask for a review - if that fails - an email to the riak-users mailing list should bring get some attention .