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.
Merge pull request #241 from dhermes/docstrings-storage

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