Add get_ref() method to DOMString
Handle style attributes in element setter instead of html parser
Call old Element::{get, set}_attr() from new Element::{Get, Set}Attribute()
auto merge of #719 : ILyoan/servo/style_setter, r=metajack

This fixes issue #699. tweaks a test case.

The last commit implements new {Set,Get}Attribute() by calling old {get, set}_attr(). but I'm not sure if this is the right approach. Should I reimplement the body of the functions? 
@metajack I think you might have an opinions for this.