RFC: Fix Windows symlink issues

Since I provided the original PR, I thought I should try to provide the fix.  

* Provide a message on Windows if symlink creation fails, indicating
  that symlinks require admin priveleges
* Automatically attempts to create junction points whenever symlinking
  a directory

I don't have easy access to a Windows machine, so could someone with a Windows machine (@quinnj, @tkelman) please test?

This may not be the ideal fix, so feedback is welcome (and feel free to modify this repo directly, as I may not get to it quickly during the week).  

Cc: @StefanKarpinski 

Ref: #4396 
warn_once?