Status Reporter

Extend YCSB client StatusThread to support user plugins to handle interval statistics. If 'statusreporter' param is set to a Java Class implementing the com.yahoo.ycsb.statusreporter.StatusReporter interface, the impl will be invoked at the end of each sampling interval and passed the stats for that interval.  Provided an example implementation HttpStatusReporter that posts the data to an Http endpoint for monitoring purposes.

Also: added parameter 'statusinterval' to override default 10 second sampling interval.

Also: eliminated multiple calls to Measurements.getSummary(). This call resets counters, so if -s (status monitoring) is enabled results were being split between stderr and stdout, with neither providing a fully accurate picture.