Port updates from original INAppStoreWindow

[zone117x]

There have been changes to the original project https://github.com/indragiek/INAppStoreWindow Most noticeably there are issues with running this older C# port on Yosemite. I've done a pull request with my refactoring of this project to use the newer Xamarin framework, but I'm not familiar enough with obj-c to port the changes. Would be great if the project maintainer could do so!