- lib/cask/url.rb: initialize(): add new `:data` attribute to hold post parameters
- lib/cask/download.rb: perform(): dispatch to new class based on `:using => :post`
- lib/cask/download_strategy.rb: Cask::CurlPostDownloadStrategy:
extend curl_args with x-www-form-urlencoded data
- doc/CASK_LANGUAGE_REFERENCE.md: HTTP URLs: document new strategy
- test/cask/test_download_strategy.rb: Cask::CurlPostDownloadStrategy: test new 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