require Global.app to be set before creating a GlobalID
add :app option
ensure GlobalID.app is set back
test situations when app is nil or GlobalID is nil
use fetch to get the option with a default
make the error message more clear
Merge pull request #12 from tony612/require-an-app-to-be-set-before-creating-a-globalid

require Global.app to be set before creating a GlobalID