Fix scheduler email setup bug, JWT authentication, plumber support

• Remove checking for existance of cloudscheduler.serviceAgent (#89 - thanks @BillPetti)
• Setting env vars for Cloud Run runtime deployments fixed
• Added cr_jwt_create() and family to create JWTs to call authenticated services such as Cloud Run (#91)
• Add authenticated Cloud Run use case
• Ensure timeout is under 86400 secs (24hrs) in cr_build_make()
• Add cr_buildtrigger_copy()
• Extra checks for existance of valid auth file when creating build email (#89)
• We can't check for existance of cloud build email #94 so cr_setup() will only set roles in assumed present Google service emails.
• Include plumber in Depends as its needed for most applications - makes FROM gcr.io/gcer-public/googlecloudrunner:master more useful in Docker files.
• Update example plumber deployment to use Rscript to start plumber server (#97)
• Add R to git use case example
• Improve cr_setup() for buckets