Fix S3 uploading once again.

You can't assume that every url returned from ckan api is relative to
the api itself. Now we try to figure out what url is the right one, and
urljoin does that like a charm.

Second, we can't send any ckan-specific headers to S3. That can break S3
upload in more ways then i can imagine.

Fixes #19
Merge pull request #21 from glance-/bugfix-s3-upload-again

Fix S3 uploading once again.