COLLADAMax - Better XRef support & more fixes of inappropriate use of toUTF8String.

XRef support has been improved. It could use some more work eg. url in <instance_node> and output path of XRefed files should be configurable but that would require development of some new UI... My changes were tested on 3DS MAX 2013 SDK and Visual Studio 2012.

PS
As a bonus, toUTF8String got removed from all the places I could find. You can discard my previous pull request as it this one includes all the changes from that one (including MFC dependency removal).