allow linkables to be set to none

sometimes it doesn't make sense for a cask to link anything out to the
Applications dir

Casks/adobe-air.rb

@@ -6,4 +6,6 @@ class AdobeAir < Cask
   #caveat 'You need to run {{install_path}}/AdobeAIRInstaller.app to actually install Adobe AIR'
   
   sha1 '8fbd2dffc20442903d8b51e7362a3191f39752b4'
+
+  link :app, :none
 end

lib/cask/dsl.rb

@@ -34,8 +34,12 @@ module Cask::DSL
     end
 
     def link(type, *files)
+      if files == [:none]
+        linkables[type] = []
+      else
         linkables[type] += files
       end
+    end
 
     attr_reader :sums
 

test/cask/dsl_test.rb

@@ -59,4 +59,14 @@ describe Cask::DSL do
     instance = CaskWithLinkables.new
     Array(instance.linkables[:app]).must_equal %w[Foo.app Bar.app]
   end
+
+  it "allow linkables to be set to empty" do
+    CaskWithNoLinkables = Class.new(Cask)
+    CaskWithNoLinkables.class_eval do
+      link :app, :none
+    end
+
+    instance = CaskWithNoLinkables.new
+    Array(instance.linkable_apps).must_equal %w[]
+  end
 end