bucket object does not work for cross-account reads

[jayluan]

because s3.bucket(name: string) is doing a list bucket under the hood before doing a find, it won't be able to reach any buckets in a different account given that cross account access is enabled.

https://github.com/seratch/AWScala/blob/master/src/main/scala/awscala/s3/S3.scala#L63