Fix for issue-63: Added config directive to enable/disable the MarkusLogger

This is done in reference to https://github.com/MarkUsProject/Markus/issues/63

Simply adding a check for the directive to the log method, in markus_logger.rb, would have only reduced the overhead of writing to the log file. However, there's often additional overhead involved in querying for the required information to log, which is unnecessary. This solution seeks to avoid that unnecessary querying as well.