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
Updated with suggestions from @rolandwalker
Merge pull request #6660 from rolandwalker/installer_stanza

DSL: add `installer` stanza
Karabiner 10.4.0
Thunderbird 31.2.0
Merge pull request #6704 from ahbeng/thunderbird-31.2.0

Thunderbird 31.2.0
Flash Player 15.0.0.189
Merge pull request #6705 from ahbeng/flash-player-15.0.0.189

Flash Player 15.0.0.189
Merge pull request #6703 from ahbeng/karabiner-10.4.0

Karabiner 10.4.0
Upgrade WebStorm.app to 8.0.5
Merge pull request #6708 from claui/master

Upgrade WebStorm.app to 8.0.5
Upgrade Coda 2.app to v2.5
Merge pull request #6707 from vinkla/coda

Upgrade Coda 2.app to v2.5
Merge pull request #6694 from toonetown/parallels-desktop-10

Update to install Parallels Desktop 10
Add MoneyManagerEx (MMEX.app) v1.1.0