Spurious parameter.
More idiomatic use of '__iter__'.
Add 'Bucket.iterator' method.

Exposes additional paramters for narrowing / batching key search.

Fixes #26.
Capture 'prefixes' returned in iterator response.
Update TestStorageListFiles regression to use 'bucket.iterator'.

See: https://github.com/GoogleCloudPlatform/gcloud-python/pull/359#issuecomment-62643880
Merge pull request #359 from tseaver/26-expose_bucket_iterator_w_extra_params

Fix #26: expose bucket iterator w extra params