Enable cdn_subdomain for secure url generation

Implement subdomain cycling according to this response from the
Cloudinary team:
http://support.cloudinary.com/entries/25445341-Subdomain-and-ssl-certificate

* Decompose generation of the url prefix into it's own method:
  Cloudinary::Utils.unsigned_download_url_prefix
* If using the shared cloudinary secure domain
  (res.cloudinary.com) and the cdn_subdomain option is enabled,
  use Zlib::crc32 on the source to pick a consistently numbered
  subdomain.
* Update utils_spec.rb to test case where both cdn_subdomain and
  secure options are enabled.