Add functional tests to mean-logger

This pull-request simply adds a Mocha test to verify the behavior of mean-logger. No changes to mean-logger.js.

Motivation: this is part of a slightly wider effort where I try to improve testability of mean's server.js, which may slightly expand the interface between server.js and mean-logger. Before doing any changes (which will be minimal, and will preserve backward compatibility) I wanted to make sure the current behavior of mean-logger is protected by tests.