Merge #8566: Easy to use gitian building script

3fe0b68 Set defaults to gitian defaults (Andrew Chow)
6ffd6b4 Create option to detach sign gitian builds and not commit the files in the script (Andrew Chow)
498d8da Check for OSX SDK (Andrew Chow)
eda4cfb Create an easy to use gitian building script (Andrew Chow)