Use GitHub::Markup to parse markup files

Closes #1382

spec/helpers/tree_helper_spec.rb

@@ -0,0 +1,15 @@
+require 'spec_helper'
+
+describe TreeHelper do
+  describe '#markup?' do
+    %w(mdown md markdown textile rdoc org creole mediawiki rst asciidoc pod).each do |type|
+      it "returns true for #{type} files" do
+        markup?("README.#{type}").should be_true
+      end
+    end
+
+    it "returns false when given a non-markup filename" do
+      markup?('README.rb').should_not be_true
+    end
+  end
+end