handle fully-qualified cask names, fixes #2235
form "<user>/<repo>/<cask>" may be used on cmdline, matching homebrew
- refactor Cask::Source::Tap into Cask::Source::Tapped, removing
  code, only handling unqualified cask names here now (ie no slash)
- create Cask::Source::TappedQualified, handling "<user>-<repo>/<cask>"
  form as before, and new form "<user>/<repo>/<cask>", for the case that
  the relevant tap already exists
- create Cask::Source::UntappedQualified, handling both command-line
  forms as above, but implicitly creating adding a new tap if it
  does not already exist
- add module Cask::QualifiedCaskName for utility functions on qualified
  Cask names
Merge pull request #2303 from rolandwalker/qualified_cask_names

handle fully-qualified cask names, fixes #2235