Implement document.createProcessingInstruction

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

This is a sub-task for #1428.
auto merge of #1622 : brunoabinader/servo/document-createprocessinginstruction, r=Ms2ger

PR #1620 adds ProcessingInstruction DOM interface.

Spec:
http://dom.spec.whatwg.org/#dom-document-createprocessinginstruction

This is a sub-task for #1428.