Add support for the 'rem' css unit
auto merge of #4340 : michaelwu/servo/css-rem-support, r=SimonSapin

This works on my simple test page https://people.mozilla.org/~mwu/rem.html , hopefully works on real pages too. Seems a little messy to add root_font_size directly to ComputedValues, but it didn't seem appropriate to add to the style structs.