Add cask for Apache Directory Studio.
user
```ruby
class ApacheDirectoryStudio < Cask
  url 'http://mirrors.sonic.net/apache//directory/studio/dist/2.0.0.v20130628/ApacheDirectoryStudio-macosx-x86_64-2.0.0.v20130628.dmg'
  homepage 'http://directory.apache.org/studio/'
  version '2.0.0.v20130628'
  sha256 '60914cc67f893afdc8bf537fa67a48cb7cd96357e7d28595a2c22a74a19f02e3'
  link 'Apache Directory Studio.app'
end
```
assistant