Bug fix:  sort by sample attribute not working for variant tracks.  Fix submitted as git pull request:   https://github.com/broadinstitute/IGV/pull/70.

Also, some minor cleanup of TrackLoader and TribbleFeatureSource.
Don't offer to create indexes for remote files.
Further refinements of on-the-fly indexing.   All tribble supported files are now treated <almost> uniformly.  Small files that are not indexed are just loaded.   For larger local files, defined to be > 10mb,   an option is presented to creat an index.    An index is required for VCF files > 10mb.  The requirement for other file types is set at 1GB.
IGV-2061 -- feature visibility window cannot be set to zero (i.e. no window) on VariantTrack
Refine getLength(file) method to work with ftp.
Disable "feature" that apparently was attempting to switch from displaying features to density outside the visibility window.  This was never announced or documented, and in retrospect seems like a bad idea.  It was discovered to be the source of a subtile bug in which features would disappear when zooming out.
Refactor for clarity & remove unused class
VCF track enhancements for 1KG genotypes (visibility window calc,  treatment of vcf.list files)
Watch for null parameters in FileUtils.findExecutableOnPath

This can happen sometimes in JWS launch (apparently)
Ignore a test if the required genome can't get loaded
Remove redundant comparator classes

Github #70
Merge remote-tracking branch 'upstream/master'

Conflicts:
	src/org/broad/igv/track/TrackGroup.java