sync_s3: enable syncing to other cloud providers compatible with s3

  * added '--s3host' and '--acl' options
  * instead of using key.set_acl, 'policy' keyword argument in set_contents_from_string
    was used (this approach is inspired by django-storages)
  * get_s3connection_kwargs added for easier overriding/extending
  * docs updated and extended
  * additionaly: found typos corrected