This reverts commit eb4f39f.

These are changes necessary to support the usage when only the bucket
url is known.

Also make that attribute required in the typespecs.

This is going to treat the endpoint differently depending on
bucket name. It is also going to make `:bucket` nil if we can
build a virtual-host style endpoint.

The rules are more or less the following:

* Endpoint is given
  * And bucket is an empty string (s3:///path): keep bucket nil
  * Otherwise keep both endpoint and bucket in config
* Endpoint is nil
  * Bucket has dots: keep the endpoint _without_ the bucket, fill in bucket field
  * Bucket does not have dots: keep the endpoint with bucket, bucket is nil