Add copious debugging with --debug

- add new file "cask/utils.rb" analogous to "utils.rb" in Homebrew
- define odebug and odumpcask, analogs of ohai and friends, but
  which only give output when --debug is in effect
- move the debug setting from an instance variable in Cask::CLI
  to a method Cask.debug, defined in "lib/cask/options.rb", which
  was added in #2276. (Perhaps options.rb should be merged back
  into Cask::CLI).
- sprinkle odebug statements liberally throughout the codebase
- update tests