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.
Merge pull request #1 from apartmentlist/feature/cycle-secure-domains

Enable cdn_subdomain for secure url generation
Use gsub instead of destructive gsub!

All urls would have the same (last) subdomain if the
`secure_distribution` string is edited in place.
Merge branch 'master' of https://github.com/apartmentlist/cloudinary_gem
Support for new domain sharding syntax and secure domain sharding. Support for secure_cdn_subdomain flag