Implement Node::replaceChild()

Implements Node:replaceChild() according to spec below:
http://dom.spec.whatwg.org/#concept-node-replace

Closes #1430.
auto merge of #1504 : brunoabinader/servo/node-replace-child, r=Ms2ger

Implements Node:replaceChild() according to spec below:
http://dom.spec.whatwg.org/#concept-node-replace

Closes #1430.
Flagging of blocks as fixed displays
Position fixed displays out of normal flow.
auto merge of #1440 : ibnc/servo/position_fixed, r=larsbergstrom

Actually removing from the normal flow will come later. 

There are some fixes for the positioned box offsets, like actually parsing percentages and naming consistency.
fix text-decoration property in layout
change position of setting inline_data for "<span>asdf<div>b</div></span>" case
fix line-height property