Tweak powershell download code

2 changes for style, 1 for correctness.
  1) It's not required to specify the leading "System" part of a namespace
  2) Unless interpolation is being used, best practice to always use single quotes
  3) Powershell keeps track of working directory separately from .NET. To download to the current psh directory, need to specify $pwd in the download path.
Merge pull request #178 from latkin/patch-1

Tweak powershell download code