[core] Merge Pull Request #371 from 'YCSB-hdr-default/master'

- *incompatible* Default measurement changed from histogram to hdrhistogram
  Users who want previous behavior can set the 'measurementtype' property to 'histogram'

- *incompatible* Reported 95th and 99th percentile latencies now in microseconds (previously in milliseconds)

Conflicts:
	core/src/main/java/com/yahoo/ycsb/measurements/OneMeasurementHdrHistogram.java

closes #371