factor CoverageFeature out into a separate module
fix getGlobalStats in Store/SeqFeature/Coverage
disable dijit highlighting in track pane dndsource
make GenomeView report the visible region in round coordinates only
refactor MismatchesMixin into JBrowse/Store/SeqFeature/_MismatchesMixin  and   JBrowse/View/Track/_AlignmentsMixin.  make the SNPCoverage track work with local autoscaling, and have it use it by default