script cleanup and additional options

* learned --branch: the branch to use when doing snapshots. Defaults to
  master.
* learned --destdir: let you override where the tarball will be written
  to. Defaults to /usr/local/share/make-release
* properly honor --yes (skip any question, assuming yes)
* alias -y to --yes

Change-Id: I331502f38051ea0a7274890f9f801a5a47cf7d5b