Various fixes for IE 10 and 11

Work around busted IE MutationObserver.

Setting innerHTML in IE 11 will result in removedNodes that are always childless.
This leaves no way for the CustomElements observer to call those detachedCallbacks.

Work around funky XML Namespace for unknown elements in IE

Last commit needed for passing tests in IE 11

Work around lack of prototype swizzling in IE 10

customMixin should stop on HTMLElement prototype

With be729c3, no call to customMixin will have HTMLUnknownElement.prototype in the chain, only HTMLElement.prototype