Allow using the default logger via macro

This adds some nifty PP directives to generate a macro which can take one or two arguments. With this, both these are possible:

LOG(mylogger, WARNING) << "I am logging this to the topic of mylogger";
LOG(WARNING) << "I am logging this to the default logger of the framework";

Now we only need to make up our minds about what the topic of the default logger should be (empty?) so it is convenient for end-user satellite implementations to log.

This closes #25 (closed)