added support for versioned bucket delete

./s3cmd rb -r s3://bucket -now removes the versioned objects as well

Since the bucket-list-versioned request is very similar to bucket-list I took the initiative to create a bucket_list_generic method to avoid duplicated code. The old method is still there and actually sets up the parameters and calls the new one. This affected a few constant strings (e.g response["list"] has now became response["Contents"] in some cases - it follows the names coming from the Amazon API response).

Looking forward for your feedback.

Regards,
Vasi