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