- 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
- introduce Cask::Source to encapsulate different types of loading
behind a consistent interface
- mostly this is method-level reorg, kept refactoring of the internals
of these methods to a minimum for this pass
- this is preparatory cleanup for adding the ability to represent a
cask without a backing ruby file
