Commands.Utility now needs Microsoft.CodeAnalysis.CSharp, which doesn't
explicitly target `dnxcore50`, so now Commands.Utility and the packages
which depend on it now much `import` the Portable Windows framework.
This also required adding the aspnetvnext feed.
System.Management.Automation now requires System.Diagnostics.StackTrace.
- Pester source code moved to `test/Pester`, deleted `ext-src`.
- Pester tests (.ps1 files) moved to `test/powershell`
- xUnit tests (.cs files) moved to `test/csharp`
- Third-party script test moved to `test/shebang`