Implement Initialize, Shutdown & Config to allow Serialized Mode in PCL

The purpose of this change is to implement Initialize, Shutdown and Config.
see #35.

This allows better multi threading ability on some platforms.

It can be used like this...

    var platform = new SQLite.Net.Platform.XamarinIOS.SQLitePlatformIOS();

    platform.SQLiteApi.Shutdown();
    platform.SQLiteApi.Config(SQLite.Net.Interop.ConfigOption.Serialized);
    platform.SQLiteApi.Initialize();

NotSupported exception on platforms not supporting this.