dartium 38663 (new formula)

Added suport to install the latest stable dartium browser from
https://www.dartlang.org. The browser is used to develop dart
applications and is common installed with the dart sdk provided by
dart.rb Formula
The `dart.rb` file, already in homebrew, functions much like this: point to a zip of binaries. Since dartium (this package) is just Chromium, I don't think we want to compile it on a user's machine (will take a very long time).

What are the differences between homebrew-binary and homebrew-cask ?