Issue #173; add properties to control logging:

- added 'nl.lxtreme.ols.logLevel' (= 0..6) to control the log level
  that should be logged;
- added 'nl.lxtreme.ols.filterJdkUiLogs' (= true/false) to control
  whether or not UI-related log messages from the JDK are to be
  logged (gives much noise with logLevel > 3);
- make sure the console logger adheres the log level supplied by
  the system property.