Document how to create bucket with predefinedAcl

I've seen #304 and looked at setMetadata and still haven't figured this out. I'd really just like to set my files to be publically readable but can't seem to figure it out :)

More specifically I'd like all the files I upload to have a checkbox on "Shared publicly"

This should definitely be documented better. Bucket objects have a special yourBucket.acl.default object of functions just like yourBucket.acl (add, delete, get, and update). Here's how you can do it:

yourBucket.acl.default.add({
  scope: "allUsers",
  role: gcloud.storage.acl.READER_ROLE
}, function (err) {})

allUsers comes right from the API: https://cloud.google.com/storage/docs/json_api/v1/defaultObjectAccessControls - instead of leaving that an impossible mystery to solve, I'll add that to the docs tomorrow :)