Need to tweak a few things in the metrics package to support the Simon plugin

(1) added an extra metrics.jar target to the build.xml so that I can build a stand-alone library containing only the
metrics package and its subpackages.

(2) added serialversionUIDs to a bunch of classes to make Eclipse happy

(3) made AbstractMetricsContext.createRecord final, and added a protected newRecord that subclasses can use
to customize record creation without breaking the parent class.

(4) minor fix to how errors in callbacks are handled

(5) constructor in MetricsRecordImpl made protected rather than package private so that it can be subclassed

(6) extended Util.parse(String serverSpecs, int defaultPort) to handle the case of a null serverSpecs by defaulting to localhost