Refactor from global variable to RenderSettings

This uses `bpy.types.RenderSettings to store data`, eliminating the #XXX UGLY
marker previously required. This is the recommended method according to [this BlenderArtists thread](http://www.blender.org/forum/viewtopic.php?t=27291&sid=20cbbe81c75ff34fae56d48d8c0670ca).

I have tested this on my local machine and it works as intended.