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
document `appcast` stanza

The :latest_version key stays undocumented, as it is not
clear yet whether it is useful
appcast :format in epub-to-pdf
appcast :format in xamarin-android
appcast :format in xamarin-ios
appcast :format in xamarin-studio
Specify that Back in Time is commercial software
Specify that Backblaze Downloader is commercial software
Specify that Backblaze is commercial software
Specify that backtobed is gratis (and fix the download link)

Previous download link didn't work because they moved the archive.
Initial version of the game is still free, and now there is another one,
paid and distributed over Steam.

The free version is called "Student version".
It doesn't seem to be open-source though.
Specify that Backup Loupe is commercial software

See http://sites.fastspring.com/somazone/product/shop?source=soma-zone
Specify that Bahamut is under MIT license

See the last section of
https://github.com/sdegutis/bahamut/blob/master/README.md
Specify that Baidu Cloud is gratis

May be inaccurate. Although I used Google Translate and it seems to be gratis.
Specify that Baidu Input is gratis
Specify that Baidu Music is gratis
Specify that Balsamiq is commercial

From their site:

> The trial is fully functional for 7 days. Afterwards, you'll need a
> key to save your work. By installing the software you are agreeing to
> Balsamiq’s End User License Agreement.