changed symbology to ugly red clickable squares
tweaked symbol and added doc comments
minor rearrangement
added verifyOffline method
added download tiles button
add offlineTileEnabler module
added verifyOnline method
moved all map initialization code to startMap()
added getMinMaxZoom()
added initOfflineTileEnabler()
added updateOfflineUsage()
verify that initOfflineTileEnabler() is working.
added downloadTiles(). Reorg'd variables.
rearranged variables again. Organized better.
added reportProgress()
download tiles is now working.
minor console.log tweak
adjusted button text