Detect and use installed 7-Zip if available.

Added function Acquire-7z which checks for the presence of a 7-Zip
installation via the registry, and falls back to downloading &
extracting 7za if one isn't found.
Minor beautification of braces and whitespace.

^
Use Join-Path when checking for a 7-Zip install.
Merge pull request #5 from whipsch/master

Detect and use installed 7-Zip if available.