Refactor TextRenderBox to handle range and run data and let a RenderBox's style handle font size
auto merge of #485 : oncemoreification/servo/refactor_TextRenderBox, r=jdm

This removes the TextBoxData struct and places its properties directly on the TextRenderBox. 

It also removes the font_size property from RenderBox's, in the thinking that this should be (and is) managed by stylers.