doc(CompareAndDelete): Add missing CAD docs

Fixes issue #549
doc(CompareAndSwap): clarify prevIndex in CAS
chore(gofmt): gofmt compare_and_delete_command.go
feat(scripts): use zip for windows and darwin

zip files are more common on OSX and windows. Use those file formats for
those two platforms.
Merge pull request #551 from bcwaldon/doc-compareAndDelete

Document CompareAndDelete
Merge remote-tracking branch 'upstream/master'

* upstream/master:
  feat(scripts): use zip for windows and darwin
  chore(gofmt): gofmt compare_and_delete_command.go
  doc(CompareAndSwap): clarify prevIndex in CAS
  doc(CompareAndDelete): Add missing CAD docs