Add underline to layout and gfx.
Added underline test
Fix rebase issues
Clean up code a little
Fix style issues
Better propagation of text-decoration, though still not perfect. Also, boxes now cannot be merged if they have different text-decorations.
Removed unnecessary FIXME
auto merge of #481 : eric93/servo/underline_tweak, r=metajack

Fix style problems with last PR. Also better propagation of text-decoration.