- move the interface from top-level methods to hash arguments of URL to
keep the cask DSL as skinny as possible
- promote the Cask::Headers object up to a Cask::URL object that
encapsulates all infornation about the URL
- pull all knowledge about curl arguments into the DownloadStrategy,
leaving URL to act as a value object to be queried for details
- test at the DownloadStrategy level; setting up expected curl args
and example casks