fix kobo frozen when exiting koreader
This should fix #509.
Merge pull request #526 from chrox/master

fix kobo frozen when exiting koreader.

It doesn't work sensibly anyway. It should have taken a screenshot of the homepage of nickel. Instead it gives the last screen before koreader, which is the png-file of koreader. 
Just quitting the png-file and returning to Nickel home screen would be sufficient. No idea how to do that, though.