updated default system tree location to 'program files', so it is protected with admin rights.
Implemented a /SELFCONTAINED installation option that makes a single directory installation
specify error exitcodes, specifically 'permission denied'.
updated comments for exitcodes
batch wrapper script (luarocks.bat) updated to auto-elevate if permission was denied
Merge pull request #211 from Tieske/exitcodes

Win defaults, /SELFCONTAINED option