Add DOMImplementation skeleton

Creates a DOMImplementation struct corresponding to DOMImplementation
WebIDL. Also implements a getter for Document::implementation.

Closes #1486.
auto merge of #1488 : brunoabinader/servo/document-implementation, r=Ms2ger

Creates a DOMImplementation struct corresponding to DOMImplementation
WebIDL. Also implements a getter for Document::implementation.

Closes #1486.