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

Similar to https://github.com/cloudinary/cloudinary_gem/pull/87 but this limits subdomain cycling to usage of the shared_domain.

* 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.