IE 11 Support

[ksons]

[csvurt]

IE 11 support should be complete now. There's one issue left (and as a result one failed test), listeners of the "onload" event added to the XML3D element as an attribute will be called twice in XHTML documents. Because we plan to phase out XHTML in the future, and because this only affects IE 11 in a non-critical way, we've decided to leave this as a known issue rather than build in an IE specific workaround.