improve some error handling
stray debugging code
remove trailing commas in jbrowse_conf.json
smooth over html-layout percentage inaccuracies in Safari (and probably other browsers) by forcing rubber-band zooms to use one of the pre-defined zoom levels
bugfixes for IE 8 support
improve phrasing of error messages when browsers do not support typed arrays
port CanvasFeatures track base class to use the new JBrowse/has