Numerous additions to "brew cask doctor"

New versions, paths, environment variables, and tests.
Side effect: release version must be stored in two locations.

There is an alternate method to get the `homebrew-cask` release version number in doctor without storing it in two locations in code:  Do something like
```ruby
require "#{HOMEBREW_LIBRARY}/Formula/brew-cask.rb"
```
in `doctor.rb`.  My reasoning in deciding against that was: in the "doctor" scenario, when things might be going wrong for the user, that `require` is capable of pulling in the wrong file.  However, storing the version number in two places is undesirable, so criticism is certainly welcomed.