Added CollectionFilter, CollectionTypeId
Implemented HTMLCollection::create_live
Support live HTMLCollection for Document collections

The following interface methods now return live collections:
Document.{images,embeds,plugins,links,forms,scripts,anchors,applets}
Support live HTMLCollection for FieldSet.elements
Support live HTMLCollection for DataListElement.options
Updated HTMLCollection content test
HTMLCollection::create returns live collections by default
Implemented Document.importNode

Spec:
http://dom.spec.whatwg.org/#dom-document-importnode
Implemented Node.adoptNode

Spec:
http://dom.spec.whatwg.org/#dom-document-adoptnode
Cleaned up recursive code in Node.clone
Added Document.adoptNode content tests
Added Document.importNode content tests
auto merge of #1932 : brunoabinader/servo/document-import-adopt-node, r=Ms2ger

Specs:
http://dom.spec.whatwg.org/#dom-document-importnode
http://dom.spec.whatwg.org/#dom-document-adoptnode
auto merge of #1952 : brunoabinader/servo/htmlcollection-live, r=Ms2ger

Closes #1662.

PS: Cached collections will be next step.
Move inline stylesheet parsing out of HTML parser.

Instead, use shared code to parse stylesheet content when it is inserted,
whether during parsing or dynamically by script.

Based on work by sanools in #1350.