add new CurlPostDownloadStrategy

- 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