Add CORS support for IE with XDomainRequest.

This adds cross domain compatibility for IE 8 and 9, which must use a special proprietary function called XDomainRequest to properly use CORS.

There are many forks of the requirejs/text project that exist solely to add in this functionality. Please, please even if you do not accept this exact PR look into adding similar support. RequireJS says it's compatible with IE6+, so it seems out of place to not have support for IE 8 and 9 CORS in a top-tier RequireJS plugin like this.