Add style structs.
Handle 'color: currentColor'
Resolve 'unset' to 'initial' or 'inherit' when parsing.
Add cascading.
Fix SimonSapin/servo-style#2: failed assertion in selector parsing.

Add a test: "parse bootstrap.css without failing"
Merge commit '25d6448f2804b5255dcdef00753cd813717424ef'
New style sytsem: add selector matching

Also make scribt::dom::element::Element::get_attr ASCII case-insensitive
on attribute names, per spec:
http://dom.spec.whatwg.org/#dom-element-getattribute
Fix missing license header.
Sorting on specificity needs to be stable for correctness of the cascade.
Selector matching: tweak variable naming, add comments.
auto merge of #1001 : SimonSapin/servo/newnewcss, r=kmcallister

Add selector matching, style structs, cascading. The matching is quite naive and has many low hanging fruits for optimization. No pseudo-class (except `:not()`) is implemented yet, but those are easy to add as needed.

Next step is to update the layout code to use this and drop netsurf-css.

(Most of the +7066 lines stat is for bootstrap.css, added as a test.)