Implement document.characterSet

Spec:
http://dom.spec.whatwg.org/#dom-document-characterset

This is a sub-task for #1428.
auto merge of #1604 : brunoabinader/servo/document-characterset, r=Ms2ger

Spec:
http://dom.spec.whatwg.org/#dom-document-characterset

This is a sub-task for #1428.