Implement document.URL & document.documentURI

Specs:
http://dom.spec.whatwg.org/#dom-document-url
http://dom.spec.whatwg.org/#dom-document-documenturi

This is a sub-task for #1428.
auto merge of #1583 : brunoabinader/servo/document-url, r=Ms2ger

Specs:
http://dom.spec.whatwg.org/#dom-document-url
http://dom.spec.whatwg.org/#dom-document-documenturi

This is a sub-task for #1428.
auto merge of #1564 : pcwalton/servo/small-vector, r=larsbergstrom

12% improvement on style recalc for the rainbow page.

r? @larsbergstrom
util: Implement a Doug Lea-style concurrent hash table.

This will be used for leaf sets and interned strings.