Replace MERGE with a much simpler (but equivalent algorithm.)

The description of the algorithm in
http://www.cs.ubc.ca/~xujian/paper/quant.pdf is much simpler and clearer
than the one I originally tried to implement, described at
http://www.mathcs.emory.edu/~cheung/Courses/584-StreamDB/Syllabus/08-Quantile/Greenwald-D.html

More important, this one (algorithm/implementation) actually appears to work \o/