ZipChanges Step 1 - Change entire website to use INupkg instead of IPackage. Build succeeds. Many tests are broken.
Fix all the unit tests that need fixes from the test side.
Implement The Nupkg class and update constructions.
Fix the failing unit tests and add new NupkgFacts unit tests.
Satisfy code analysis
Incorporate CR feedback, and new learnings about zip file format.
Bug fixes from testing. Mostly around stream disposal timing. Also added error handling for invalid packages on the upload page, and an error recovery page 'UnverifiablePackage' to handle the rare case where you cleverly get an invalid file saved in the uploaded file cache.
Merge branch 'tilovell-837' into mergezip

Conflicts:
	Website/Services/NuGetExeDownloaderService.cs
	Website/Services/PackageService.cs