* convert existing Cask:: namespace to Hbc::
* move Homebrew-fork code under Hbc::
* move freestanding classes such as Tty and TopologicalHash under Hbc::
* recast HOMEBREW_CASK_ constants as HBC_
* modify our Homebrew Formula for backward compatibility
* devscripts and dev docs
- do not remove *all* symlinks from referenced dirs, only broken ones
- restore dir permission after use so we don't leave behind 777 dirs
- add some better testing around `Cask::Pkg`
- clean up `FakeSystemCommand` interface in tests
refs #1274