fixed pinpointer registration in QualityAssessment

- only the class name was considered when deciding whether to register a
  pinpointer or not
- now all the base class names are considered
added writeToSink method in metrics base class

- added the method writeToSink to the MetricImpl class to avoid
  redundant code in all the metric implementations
- this method is a first shot approach and should not last forever;
  suffices since the interface to the MeasureDataSink is not clear, yet
configured threshold now used to decide if URL too long

- the ShortUrl metric now uses the threshold defined in the
  metrics.properties file to decide if a URL is considered too long or
  not