Let there be color

### Color and Highlighting!

Color-code the `brew update` and  `brew ls` to quickly see which formulae, have changed and which are outdated.
Do this only on a Tty, so your scripts are fine! I think this highlighting and mentioning of `brew upgrade` also solves the flaw that people do think they are up-to-date when they did a `brew update`.

`brew deps <formula>` and `brew uses <formula>` now also marks installed dependencies and `brew deps <formula` additionally prints missing deps in red.
(Wich `brew deps` also indicating unlinked deps that are not keg_only).

I introduce a class `Rack` to simplify things. This has been discussed throughout the different versions of this PR. I tried to keep the changes at a minimum level. I will make another pull request in another branch where this new Rack class is used throughout the homebrew code-base to get rid of repeated code blocks dealing with verification of racks and kegs.

Wanna test/see in action?
```
cd $(brew --prefix)/Library/Homebrew/tests
ruby test_updater.rb -- --verbose
ruby test_utils.rb -- --verbose
ruby test_rack.rb
```

See commit message for details.