exclude __MACOSX dir when unzipping

should take care of #222

lib/cask/installer.rb

@@ -63,7 +63,7 @@ class Cask::Installer
       elsif _zip?(path)
         destdir = "/tmp/brewcask_#{@title}_extracted"
         `mkdir -p '#{destdir}'`
-        `unzip -d '#{destdir}' '#{path}'`
+        `unzip -d '#{destdir}' '#{path}' -x '__MACOSX/*'`
         begin
           yield destdir
         ensure

test/cask/installer_test.rb

@@ -61,6 +61,14 @@ describe Cask::Installer do
       }, /Here are some things you might want to know/)
       with_caveats.must_be :installed?
     end
+
+    it "does not extract __MACOSX directories from zips" do
+      with_macosx_dir = Cask.load('with-macosx-dir')
+      shutup do
+        Cask::Installer.install(with_macosx_dir)
+      end
+      with_macosx_dir.destination_path.join('__MACOSX').wont_be :directory?
+    end
   end
 
   describe "uninstall" do

test/support/Casks/with-macosx-dir.rb

@@ -0,0 +1,6 @@
+class WithMacosxDir < TestCask
+  url TestHelper.local_binary('MyFancyApp.zip')
+  homepage 'http://example.com/MyFancyApp'
+  version '1.2.3'
+  sha1 'b0475c34136c8e00ceb4b95c245291d53d5deab3'
+end