adjust nice value of reader.lua process to -2

when launching koreader with KUAL
This should fix #413.

extensions/koreader/menu.json

@@ -7,24 +7,24 @@
 		{
 			"name": "Start the filemanager",
 			"priority": 1,
-			"action": "/mnt/us/koreader/koreader.sh",
+			"action": "nice -n -7 /mnt/us/koreader/koreader.sh",
 			"params": "/mnt/us/documents"
 		},
 		{
 			"name": "Open the last document",
 			"priority": 2,
-			"action": "/mnt/us/koreader/koreader.sh"
+			"action": "nice -n -7 /mnt/us/koreader/koreader.sh"
 		},
 		{
 			"name": "Start the filemanager (no framework)",
 			"priority": 3,
-			"action": "/mnt/us/koreader/koreader.sh",
+			"action": "nice -n -7 /mnt/us/koreader/koreader.sh",
 			"params": "--framework_stop /mnt/us/documents"
 		},
 		{
 			"name": "Open the last document (no framework)",
 			"priority": 4,
-			"action": "/mnt/us/koreader/koreader.sh",
+			"action": "nice -n -7 /mnt/us/koreader/koreader.sh",
 			"params": "--framework_stop"
 		}
 		]