Don't require creds for upload.createupload.

Allows `createupload` to work with any given S3 URL, not just one
returned in a creds object.
Merge branch 'master' of github.com:mapbox/mapbox-upload into no-creds-for-createupload
Merge pull request #16 from mapbox/no-creds-for-createupload

Don't require creds for upload.createupload.