add on_uninstall callback at formula

Let's see the rbenv formula

```ruby
require 'formula'

class Rbenv < Formula
  homepage 'https://github.com/sstephenson/rbenv'
  url 'https://github.com/sstephenson/rbenv/tarball/v0.4.0'
  sha1 'a5e80249f985294c1c9f0914f7cbdc85d4cadd74'

  head 'https://github.com/sstephenson/rbenv.git'

  def install
    prefix.install Dir['*']

    var_lib = "#{HOMEBREW_PREFIX}/var/lib/rbenv/"
    ['plugins', 'versions'].each do |dir|
      var_dir = "#{var_lib}/#{dir}"
      mkdir_p var_dir
      ln_sf var_dir, "#{prefix}/#{dir}"
    end
  end

  def caveats; <<-EOS.undent
    To enable shims and autocompletion add to your profile:
      if which rbenv > /dev/null; then eval "$(rbenv init -)"; fi

    To use Homebrew's directories rather than ~/.rbenv add to your profile:
      export RBENV_ROOT=#{opt_prefix}
    EOS
  end
end
```

On install process, it creates directories at ```HOMEBREW_PREFIX``` (/usr/local as default) directory. So, if you uninstall rbenv ```brew uninstall rbenv```, your environment is dirty. For this case, my propose is to create an ```on_uninstall``` method into formula, like:

__WARNING: The below code may not be working.__

```ruby
require 'formula'

class Rbenv < Formula
  homepage 'https://github.com/sstephenson/rbenv'
  # ...

  def on_uninstall
    var_lib = "#{HOMEBREW_PREFIX}/var/lib/rbenv/"
    puts "  removing #{var_lib} directory"
    ['plugins', 'versions'].each do |dir|
      var_dir = "#{var_lib}/#{dir}"
      rm_rf "#{prefix}/#{dir}"
    end
    rm_rf var_lib
  end
end
```