Parse `min-width` and `max-width`
Implement max-width, min-width for BlockFlow.
auto merge of #1150 : ILyoan/servo/max_width, r=ILyoan,metajack

For #783, only for block flow.
I'll do the things for other flows as well.

r? @simonsapin, @metajack
Add :any-link (Level 4), :link and :visited selectors.

All links are considered "unvisited" for now.
Remove unneeded clones.
auto merge of #1202 : SimonSapin/servo/link-selectors, r=jdm

All links are considered "unvisited" for now.

In particular, this fixes the UA stylesheet rule that underlines links.
auto merge of #1204 : jdm/servo/clones, r=kmcallister

r? @kmcallister