Basic Contacts mocking

Get the contacts app to work in desktop FF as well. How nice. This requires mozilla-b2g/gaia#8712 to have landed as it exposes some timing issues in that app at the moment.

Very basic work so far, just needed something for @OlavHN to get started.

Please note that I only mocked mozContacts here, the other contact related APIs should work well in nightly I think.