Fixes #536 - add download_url to /universe

When retrieving the universe from supermarket, it should contain
`download_url` - the full URL for downloading the cookbook. This
restores the full URL from `location_path` prior to PR #520, while
maintaining compatibility with berkshelf.

* implement download_url data for /universe
  - renames #download_path to #location_path
  - add #download_url method and constant
  - add #protocol_host_port method as it will be used by #download_url
    and #location_path
* use example.com like the RFC suggests; we dont own narf.com :)
Can we compute this stuff outside of the loop, save it to a variable, and then use it. It seems like this is static data that does not depend on the actual cookbook being processes.