- work around Mono bug in GZipStream (see http://sourceforge.net/p/keepass/bugs/1373/) by including a custom GZipStream implementation
- fixed issue with export database to System file picker location
- fixed issues with preference options crashing with ActivityNotFound (due to changes in Xamarin 5.1, see https://bugzilla.xamarin.com/show_bug.cgi?id=33617
updated manifest/changelog