Issue 1640 - Move sync account from menu to preferences

As per [Issue 1640](http://code.google.com/p/ankidroid/issues/detail?id=1640), this removes the menu option "Sync account" and instead places it in Preferences -> General -> Synchronization. 

I don't know if launching an activity from the preferences screen is a bad idea. I'm still new to this Android stuff, so any feedback is appreciated.