* contain logic within module Hbc::MacOS
* add release_with_patchlevel method
* allow MACOS_RELEASE and MACOS_RELEASE_WITH_PATCHLEVEL
environment variables to override the determined release
values regardless of whether under the test harness
* always coerce `:macos` value to array; simplify code
* remove warning comments about unset values
* doc explicitly that requirements are covered as well as dependencies
* doc that `depends_on` is not consulted until `install` time
* in docs and code, say "release" instead of "version" for the :macos release
* todo: provide our own variable styled "release" to use in Casks, instead of Homebrew's `MacOS.Version`
* doc that symbols are preferred for `:macos` over strings in Casks (interally, all are converted to idential `Gem::Version` objects)
* doc: remove `os_version_only` from `cask_language_deltas.md`
* doc: improve internal links
* doc: recast OS X release symbol list as table
* doc: (incidental) regularize orthography of Formula and Cask
Vítor Galvão