Refactored UnzipModule w/ several improvements

1. Now supports unzipping files using the System.IO.Compression.ZipFile
   class. This is present in .NET 4.5 and has the benefit of working on
   Server Core installations of Windows where it's installed. This way,
   PsGet can work on Server Core installs without the Windows Shell.
2. Automatically attempts to use Windows Shell Zip integration to unzip
   and tries the .NET 4.5 method if unsuccessful. If both mechanisms are
   unsuccessful we can provide some basic advice on how to fix the issue.
3. Some miscellaneous minor fixes and code cleanup :)
Quick spelling fix
Improve the UnzipModule failure remediation info
Merge pull request #115 from ralish/improved-unzip

Windows Server Core support in UnzipModule