Renamed 'unstable' to 'beta'.

Our dev processes are pretty darn good now, and we have a lot more
eyes on code. I'd like to change `unstable` releases to `beta` releases.
(It's good enough for Google and Squad!)
Merge pull request #594 from pjf/beta_than_anything

Renamed 'unstable' to 'beta'.
Merge branch 'release-v1.2'

* release-v1.2:
  Renamed 'unstable' to 'beta'.