ga-beacon: delay urlfetch to GA
Merge branch 'patch-1' of https://github.com/proppy/ga-beacon
log GA hit in the background (via GAE task)

- https://developers.google.com/appengine/docs/go/taskqueue/delay
- https://developers.google.com/appengine/docs/quotas#Task_Queue
don't panic in request path / if UUID generation failed