Update readerscreenshot.lua

This adds screenshot functionality for the Kobo Aura. 

The pam-files generated can be read and converted using Imagemagick for linux/windows.

~~I've added the infomessage because it takes 5+ tries to get a screenshot by tapping the lower left and upper right corners. The other diagonal is not working for the Kobo Aura yet. ~~
I've debugged the issue and it appears _I have_ to be faster to generate a screenshot. Both diagonals are working. 

But I must say that multi-touch on Kobo Aura is still a hassle.