Provide an "inplace subscription" API

[julien51]

There is still one extra non required step as we still redirect the user to the reader to perform the susbcription.
An 'in place' subscription would be quite convenient. When the user clicks the button, if that service supports in place, the button will show a 'loading' message while it tries to perform the subscription. When the subscription works, it will show a mark, if not the user will be redirected to the reader.