"Previous" button of the browser should go back to paper view

[nud]

After clicking on an article to read it, there is no obvious way to get back to the view with the paper pages. My first try was to hit the "previous" button as this what usually "works" to go back.

[diox]

Back button is going to be tricky, but there should be a close button just like every other popin, I just forgot it :)

[nud]

well, gmail does it, so you should be able to do it as well ;-)

[diox]

See #19

[lauxley]

We can close this once #25 gets merged.