Add DownloadStrategy and Cask::Headers for custom :headers support
accepts a :user_agent argument and/or multiple :cookies
  headers :user_agent => 'Netscape/1.0', :cookies => { :cookey => "r00t" }

Also adds a standalone :fake_user_agent for a default UserAgent (Chrome)
  fake_user_agent
References #1175, #958
Merge pull request #1944 from jonahoffline/add_headers_download_strategy

Add DownloadStrategy with custom :headers support
some refactoring around the new download strategy

 - 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
Merge pull request #2107 from phinze/headers-integration

Headers integration