Prepare to replace music/sound effects impl with deferred loading audio sources.
 - Clean up usage of music/sound in gurk/mapview
Clean up audio loading with an AudioResource class.
 - Reduce main page load size from ~7MB (~4MB to load) down to ~300KB.
 - Don't embed any audio elements in page.  Request them through the ResourceLoader class when needed.
 - Remove GURK from splash screen.
GZip page assets with express.compress middleware.
Merge branch 'jdd-audio-loading'
Fix issue with activeTrack and music not ending when leaving battle with certain config options.
Use camera position for feature visibility checking against shadows grid.
   cliprect points will not be < 0, so features on the extremes could end up being shadowed improperly.