Changed the target fx of NuGet.Packaging from PCL to net45
Eventually, it should target both net45 and CoreCLR
Removed IFileSystem abstraction and simply using ZipArchive in its place
Implemented the default implementation of PackageExtractor

            // TODO: Need to handle PackageSaveMode
            // TODO: Need to handle satellite package files differently
            // TODO: Support overwriting files also?
Implemented PackageSaveMode to also store nupkg or nuspec as needed.
Need to resolve a couple of issues with directory names for portable
frameworks and not extracting '[ContentTypes].xml'
Merge pull request #1 from NuGet/daravind/PackageExtractor

Daravind/package extractor