Update gen.sh to use functions, include more comments, and put a few more things into variables
Add option to re-download SteamOSInstaller.zip

Realized that gen.sh would continue to use the existing .zip, which
could cause issues if a newer one was available upstream. Gives the user
and option to force a re-download.
Merge in upstream master to update pull request https://github.com/directhex/steamos-installer/pull/17
Fix up some vars that were missed in the merge
Merge pull request #17 from ecliptik/updategensh

Update gen.sh to use functions