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.

@jasonzhao6 @quarterdome @kristjan @kkorach