PowerShell script for Windows

[angelsl]

I made a small PowerShell script for myself to grab the latest nightly and install it.

@retep998 suggested I submit it here, though I'm a bit reluctant to do so in its current state:

• It depends on 7z existing somewhere
• It depends on the structure of the tar files not changing

and finally

• PowerShell sucks, because by default PowerShell script execution is disabled. One could enable it in that shell session by Set-ExecutionPolicy Unrestricted Process though.