add Octokit.delete_ref method and specs

lib/octokit/client/refs.rb

@@ -65,6 +65,18 @@ def update_ref(repo, ref, sha, force=true, options={})
         patch("repos/#{Repository.new(repo)}/git/refs/#{ref}", options.merge(parameters))
       end
 
+      # Delete a single reference
+      #
+      # @param repo [String, Repository, Hash] A GitHub repository
+      # @param ref [String] The ref, e.g. <tt>tags/v0.0.3</tt>
+      # @return [Response] A response object with status
+      # @see http://developer.github.com/v3/git/refs/
+      # @example Delete tags/v0.0.3 for sferik/rails_admin
+      #   Octokit.delete_ref("sferik/rails_admin","tags/v0.0.3")
+      def delete_ref(repo, ref, options={})
+        delete("/repos/#{Repository.new(repo)}/git/refs/#{ref}", options, 3, true, true)
+      end
+
     end
   end
 end

spec/octokit/client/refs_spec.rb

@@ -64,5 +64,16 @@
     end
   end
 
+  describe ".delete_ref" do
+
+    it "should delete an existing ref" do
+      stub_delete("/repos/octocat/Hello-World/git/refs/heads/feature-a").
+        to_return(:status => 204)
+      ref = @client.delete_ref("octocat/Hello-World", "heads/feature-a")
+      ref.status.should == 204
+    end
+
+  end
+
 end