Out of the Cellar, into the Caskroom

Install casks into HOMEBREW_PREFIX/"Caskroom" instead of the Cellar.
This prevents us from colliding with normal Homebrew Formulae.

NOTE: this will be a breaking change, with the following effects:

- all installed casks will no longer be reported as installed
- orphans all existing casks in the Cellar
- no automated facility for cleanup or migration of existing casks

refs #38, where we can discuss the implications of this before cutting a
new release