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 :)
Oh, well would you look at that! That one option just spawned all this: https://github.com/GoogleCloudPlatform/gcloud-node/commit/bf284ec07f742c545ae02779e99e590e43ea1ce3

Still need to add tests for the new options, but wanted to let you see it in case you had other ideas for how to integrate. Docs are updated as well: