Documenting undocumented classes/methods/etc. in gcloud.storage.

Also did tiny clean-up along the way in places I noticed
code issues.

Another thing, I suggest a rename of _bytes_written in
storage.iterator.KeyDataIterator to something like
_bytes_read or _bytes_consumed, i.e. something which
actually matches the action being taken.
Should we rename it to '_KeyDataIterator' to signal non-API-ness?