Add Distribution Support to Semantic-metrics.

[ao2017]

We are adding distribution support to Heroic so we can provide stat on actual data distribution as oppose to local source stat.
Semantic Metrics is an adaptation of com.codahale.metrics so we are essentially extending that library.
We will be adding these two interfaces.

public interface Distribution extends Metric, Counting {

    /**
     * Record value from Min.Double to Max.Double.
     * @param val
     */
    void record(double val);

    /**
     * Return serialized distribution and flush.
     * @return
     */
    ByteBuffer getValueAndFlush();

}

public interface MetricRegistryListener extends com.codahale.metrics.MetricRegistryListener { // add code  with distribution support}

The actual implementation of Distribution will be a Tdigest wrapper.