Implement Element.className
implemented Node.normalize()
auto merge of #1907 : brunoabinader/servo/element-classname, r=Ms2ger

Removes stub implementation from HTMLElement & implements logic in Element WebIDL.
auto merge of #1894 : khodzha/servo/node_normalize, r=Ms2ger

To resolve issue #1823 (without DOM Range updates)
Split TCast::to into TCast::to_unchecked and TCast::to.