Additonal output stats to CollectInsertSizeMetrics.  

Small addition to the output of CollectInsertSizeMetrics.
Gives 2.5%, 25%, 75%, and 97.5% percentile values useful for making boxplots and such.

Just uses the methods already available in the Histogram class, so it is a very simple addition.
New output is at the end of the current stats fields (though before SAMPLE, LIBRARY, and READ_GROUP), so impact on tools which process the output might exist, but should be minimal.