added prefs synchronize to be sure that prefs get stored

this could configured by a config boolean flag

Although the user preferences are stored at intervals it can happen (especially during testing/debugging) that a write gets lost - resulting in interesting problems. 

In addition this the method should be called when the app is backgrounded since in iOS7.0+ user preferences are not automatically synchronise on backgrounding (not implemented in this pull request)

See iOS user preferences synchronise: 
"Writes any modifications to the persistent domains to disk and updates all unmodified persistent domains to what is on disk."
https://developer.apple.com/library/ios/documentation/cocoa/reference/foundation/Classes/NSUserDefaults_Class/Reference/Reference.html