Add support for specifying coverage paths in CommandExecutor
webstart javascript -- close to working for https
coverage path in urls  & commands
Add log.debug statement for getting system proxy

IGV-2066
Snapshot improvements

Set PNG as default file filter. More informative error message if user tries to save as EPS
webstart.js changes & fixes.   Wrapped code in an igv function object to minimize use of global namespace.
Merge remote-tracking branch: upstream/master