Implement ACL abstraction for Cloud Storage API

@ryanseys

@ryanseys' refactoring proposal contained a stub at an ACL API that we deprioritized in favor of support for more API.
The time has come to implement it. I like the gcloud-python approach, where you can do things like:

bucket.acl.user('[email protected]').grant_read()
bucket.acl.all_authenticated().grant_write()
bucket.acl.save()

@silvolu

Snipped this out from the post linked above.

ACL class

var myACL = storage.acl(['entity', 'permission', 'entity2', 'permission2']);

Buckets:

myBucket.clearDefaultACL(cb);
myBucket.getDefaultACL(cb);
myBucket.setDefaultACL(myACL, cb);

myBucket.clearACL(cb);
myBucket.getACL(cb);
myBucket.setACL(myACL, cb);

Files:

myFile.clearACL(cb);
myFile.getACL(cb);
myFile.setACL(myACL, cb);

And here's another variation to consider, which is more like @silvolu's model above. The demos below use myBucket but should work with myFile as well.

myBucket.acl.grant('[email protected]', ['read', 'write'], function(err) {});
myBucket.acl.revoke('allUsers', 'read', function(err) {});

// array variations:
myBucket.acl.grant(['[email protected]', 'someotherdude'], ['read', 'write'], function(err) {});
myBucket.acl.revoke(['[email protected]', 'allAuthenticatedUsers'], 'write', function(err) {});

myBucket.acl.get(function(err, acl) {
  acl = [
    { account: '[email protected]', permissions: ['read', 'write'] },
    { account: 'someotherdude', permissions: ['read', 'write'] }
  ]
});

myBucket.acl.get('[email protected]', function(err, acl) {
  acl = {
    account: '[email protected]', permissions: ['read', 'write']
  }
});

Maybe get/set/delete instead of get/grant/revoke?
Should we allow easier ACL setting when creating a bucket, or is letting them configure the metadata object enough?

* bump * :)

Whoops overlooked this. So just to simplify we have:

Get acl is .get([user,] cb)
Set acl is .grant(users, sharedPermissions, cb) and .revoke(users, permission, cb)

Questions:

How will be setting default permissions for new objects in a bucket?
Should allAuthenticatedUsers be abstracted to its own method call? e.g. grantAllAuthenticated() or something?
How can I make all my bucket objects public for everyone, including new objects that I might add later? (I imagine this is a common use case)

How will be setting default permissions for new objects in a bucket?

Something like:

acl = [
    { account: '[email protected]', permissions: ['read', 'write'] },
    { account: 'someotherdude', permissions: ['read', 'write'] }
  ]
myBucket.defaultAcl.set(acl, function(err, acl) {
  // whatever
});