this delegates to homebrew's uninstall to get its work done. vanilla
`brew uninstall` actually works, but this gives us a more consistent
interface.
as discussed in #47
I chose Caffeine since it's relatively small to download. Eventually I'd
like to switch this up so we bundle a dmg, zip, etc that the test suite
wires in to verify that we do the Right Thing (tm) with all those file
types. Probably will want to support `file://` URLs in Casks to help us
for that use case.
Just getting started of course, but this piggy backs on Homebrew's
testing strategy to give us a platform for a fully featured test suite.
Neato!
And the tests provide value right away, as I added some better error
handling to `Cask.load`.
Big things ahead. Just you wait.
phinze