make SNPcoverage track in volvox examples use autoscaling
make SNPCoverage track inherit from Wiggle/XYPlot, not Wiggle placeholder
tweak alignment and SNPCoverage default colors: make reference coverage a lighter gray, and make unknown bases a darker gray
tweak SNPCoverage track binning and rendering to make SNPs visible when zoomed much further out.  fixes #197
don't fetch stats for Wiggle-based tracks if they are not actually needed to calculate the vertical scale
make default _getRegionStats implementation cache/aggregate region stats results using LRUCache.  big performance boost for autoscaled tracks.