replaced 'execute' calls with direct calls to 'onProgress' in 'AssetManager.loadQueue'

That way, the method works with "...rest" arguments as well. This issue came up in the forum:
http://forum.starling-framework.org/topic/asset-manager-questions?replies=25#post-66667
enhanced "auto" profile selection so that it explicitely avoids software rendering if possible

As reported here:
http://forum.starling-framework.org/topic/starling-profile-auto-selector-bug
added 'isLoading' property to AssetManager
added 'PARSE_ERROR' event to AssetManager (closes #587)
removed obsolete code
Merge remote-tracking branch 'upstream/master'