Add support for rar archives.

Library/Homebrew/download_strategy.rb

@@ -73,6 +73,8 @@ class CurlDownloadStrategy <AbstractDownloadStrategy
       # TODO check if it's really a tar archive
       safe_system '/usr/bin/tar', 'xf', @tarball_path
       chdir
+    when 'Rar!'
+      quiet_safe_system 'unrar', 'x', {:quiet_flag => '-inul'}, @tarball_path
     else
       # we are assuming it is not an archive, use original filename
       # this behaviour is due to ScriptFileFormula expectations