Updated URLs to point at Alpha 11 URLs, reduced number of intermediate
stages, simplified logging code, and added a new %PSCORE% environment
variable to make it easier to launch from other shells
This change is made at the request of the PowerShell committee,
as the use of "stable" implied more stability than available with alpha
releases. This also changes the images from existing under one repo to
two: powershell/powershell and powershell/powershell-nightly.
The documentation and script have been updated.