storage: acls

RE: #285 

Docs:
- http://stephenplusplus.github.io/gcloud-node/#/docs/master/storage/bucket
- http://stephenplusplus.github.io/gcloud-node/#/docs/master/storage/file

---

##### bucket.acl.add(scope, role, callback)
##### bucket.acl.delete(scope, callback)
##### bucket.acl.get([user,] callback)
##### bucket.acl.update(user, scope, callback)

These methods are all available with `bucket.acl.default` as well, to act on the default object ACLs.

It didn't work as I originally thought, such as that I can update many roles at once, or add many roles to one scope. Instead of aliasing scopes like "this is an email", "this is a group", or the ones like "allAuthenticated" and "allUsers", we could just borrow the terms and schemes already in place via the API (e.g. `"user-email@email.com"`). So this is an almost transparent API passthrough, with minor massaging to the input.

If you guys like other names better than add/delete/get/update, I'm happy to update :)
See what you think about how this is presented: http://stephenplusplus.github.io/gcloud-node/#/docs/master/storage/bucket - if you still think we need to update the code examples, we can do that.