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'
auto merge of #1001 : SimonSapin/servo/newnewcss, r=kmcallister

This is still not functional, (selector matching is missing) but sending what I have so far for review before the diff gets even bigger.

(Most of the +7066 lines stat is for bootstrap.css, added as a test.)