Add a getter of 'document.head' according to HTML spec.
Add a getter and a setter of 'document.title' according to HTML spec.
Modify the test file.
auto merge of #658 : june0cho/servo/docBindings, r=jdm

Add a getter for document.head and document.title.

Like in the modified test (document.title="changed title"), setters for elements doesn't seem to be called.