Refactor FontStyle and really fix #193

* Using .connect(", ") just to use .split(",") later is silly.
Keep it a vector.
* The 'font-style' property can not be both italic and oblique.
Use an enum instead of two booleans.
Remove some trailing whitespace
auto merge of #1455 : SimonSapin/servo/font-style, r=kmcallister

* Using .connect(", ") just to use .split(",") later is silly. Keep it a vector.
* The 'font-style' property can not be both italic and oblique. Use an enum instead of two booleans.