Add support for Adobe Air app installations

Adobe Air applications require the Adobe Air runtime to be present on
the user's system prior to the installing an air cask.  Presently, if
homebrew-cask doesn't detect the Adobe Air runtime, it will provide an
informative error message and instructions to obtain it via
`brew cask install adobe-air`.  This is obviously not ideal and the
installation process for the runtime should be automated in the future.

The support for Adobe Air applications manifests itself here in the
form of a new container type that the Adobe Air runtime installs to
the Caskroom.  The name of the installed .app should be provided as a
link artifact in the cask.  Once the .app is stored in the Caskroom,
the remainder of the installation process is identical to a standard
.app installation.