mark `license :unknown` with explicit todo comment

In a [conversation elsewhere](https://github.com/Homebrew/homebrew/pull/34496#issuecomment-64772953), `license :unknown` was misinterpreted as meaning we don't care about licenses.

In fact the `license` stanza is just very new.  We have already annotated 36% of Casks in this repo with something more specific than `:unknown`.  I still feel strongly that we should continue accepting submissions that use `license :unknown`, but we should also keep moving forward toward filling in this information.

This comment seems like a reasonable step.  If accepted, I will mark up existing Casks as well.