Recast Cask::DSL::Installed as Cask::Staged

Change namespace for clarity, as we already have Cask::Installer,
and are about to add Cask::DSL::Installer.

Move to parent because these methods are not DSL-specific.
use path_environment_variable for caveats test

instead of manual_installer
recast install_script to installer :script

* document installer :script.  install_script was never documented
doc: remove needless column from table
dsl: add `installer :manual`

a toplevel artifact, intended to replace `manual_installer`,
which is currently implemented within the `caveats` mini-DSL
Update to install Parallels Desktop 10
comment out broken appcast in induction.rb
add appcast :sha256 in adapter
add appcast :sha256 in adium
add appcast :sha256 in amethyst
add appcast :sha256 in apikitchen
add appcast :sha256 in appcleaner
add appcast :sha256 in arranger
add appcast :sha256 in audiobookbinder
add appcast :sha256 in aurora
add appcast :sha256 in backuploupe
add appcast :sha256 in bbedit
add appcast :sha256 in bibdesk
add appcast :sha256 in bit-slicer
add appcast :sha256 in black-ink