Fix #1435: em units in font-size

Font-based units in the font-size property refer to the font-size
of the parent, not itself.
Add a test for percentages in font-size
auto merge of #1436 : SimonSapin/servo/font-size-em, r=kmcallister

Font-based units in the font-size property refer to the font-size of the parent, not itself.
Pass a Namespace to Element::get_attribute.
Pass a Namespace to Element::get_attr.
auto merge of #1438 : Ms2ger/servo/cleanup-get_attribute, r=jdm

Passing these around as strings is an anti-pattern, IMO.