add simple sync service as a plugin

The 'KOSync' plugin will synchronize furthest reading progress
across different koreader devices after users registering their
devices.

The synchronizing service is open-sourced as the project
[koreader/koreader-sync-server](https://github.com/koreader/koreader-sync-server).

frontend/apps/reader/modules/readerrolling.lua

@@ -208,6 +208,10 @@ function ReaderRolling:onSaveSettings()
     self.ui.doc_settings:saveSetting("show_overlap_enable", self.show_overlap_enable)
 end
 
+function ReaderRolling:getLastProgress()
+    return self.xpointer
+end
+
 function ReaderRolling:addToMainMenu(tab_item_table)
     -- FIXME: repeated code with page overlap menu for readerpaging
     -- needs to keep only one copy of the logic as for the DRY principle.