Add 'subdir' as alias for 'name' in StorageObject

[icewind1991]

When listing objects with delimiter=/ the response for any sub directory will only contain {"subdir": "folder/"}

Without this alias there is no way get a list of sub directories (without listing all objects)

[haphan]

PR LGTM, however, subdir is not documented anywhere as far as I can see.

@icewind1991 Do you have any document explains this field?

Ref: https://bugs.launchpad.net/swift/+bug/1505768

[icewind1991]

I couldn't find any documentation about this, I figured it out when debugging an upgrade from the old php-opencloud which does handle it

[haphan]

thanks @icewind1991

[haphan]

This issue has been backported to branch 2.x via #218