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

The intention is to provide output which might be useful to Cask authors, to 
see where things go awry, or for users and Cask authors to produce gists
for developers.