Added seriesMedian() and seriesPercentile() statistics to the series obj...

This is a pretty straight-forward addition of seriesMedian() and seriesPercentile() to the list of statistics that can be computed for a series object.   These are useful for all sorts of things, but presently I am hoping to use them to stretch the contrast of the data before colormapping it, retaining only the data that falls between the 1% and 99% percentile, and clipping off the few outliers outside of that!

The median (but not the percentile) is also computed when you call the seriesStats() function, since computing the median is not a terribly expensive operation compared the communication overhead involved in getting the results back with Spark. 

The percentile is only available as a seriesPercentile() method, and not as part of the seriesStats(), since it requires an argument (the percentile) to work.  If desired, we could add some pre-computed percentiles (1%, 99%) to the seriesStats() method, but I think its fine to let the user call seriesPercentile() for now.