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.

frontend/ui/reader/readerscreenshot.lua

@@ -22,7 +22,18 @@ function ReaderScreenshot:init()
 end
 
 function ReaderScreenshot:onScreenshot()
-	os.execute("screenshot")
+	if Device:getModel() ~= 'Kobo_phoenix' then
+		os.execute("screenshot")
+	else Screen.bb:invert() 
+		local screenshot_name = os.date("screenshots/Screenshot_%Y-%B-%d_%Hh%M.pam")
+		UIManager:show(InfoMessage:new{
+			text = _("Writing screen to "..screenshot_name),
+			timeout = 2,
+		})
+		Screen.bb:writePAM(screenshot_name)
+		DEBUG(screenshot_name)
+		Screen.bb:invert() 
+	end
 	UIManager.full_refresh = true
 	return true
 end