half cherry-picked/half merged e625e27d from dkinzer/hells-kitchen
add proxy support for downloading tools
fix download URL for ruby (should be .7z), fix extraction for devkit (should be `7zip x` rather than `7zip e`
adapt set-env.bat to inlcude RUBYDIR and DEVKITDIR
GH-20: update README and CHANGELOG
use https:// rather than git:// URLs so that cloning works behind our firewall too
Merge branch 'use-devkit'