Autoload yaml files under _data directory

The jekyll engine will autoload all yaml files(ends with .yml or .yaml)
under _data. If there's a file members.yml under the directory, then user
can access contents of the file through site.members.

test/source/_data/languages.yml

@@ -0,0 +1,2 @@
+- java
+- ruby

test/source/_data/members.yaml

@@ -0,0 +1,7 @@
+- name: Jack
+  age: 27
+  blog: http://example.com/jack
+
+- name: John
+  age: 32
+  blog: http://example.com/john

test/source/_data/products.yml

@@ -0,0 +1 @@
+../products.yml

test/source/products.yml

@@ -0,0 +1,4 @@
+- name: sugar
+  price: 5.3
+- name: salt
+  price: 2.5

test/source/symlink-test/_data

@@ -0,0 +1 @@
+../_data

test/test_site.rb

@@ -335,5 +335,62 @@ class TestSite < Test::Unit::TestCase
       end
     end
 
+    context 'data directory' do
+      should 'auto load yaml files' do
+        site = Site.new(Jekyll.configuration)
+        site.process
+
+        file_content = YAML.safe_load_file(File.join(source_dir, '_data', 'members.yaml'))
+
+        assert_equal site.data['members'], file_content
+        assert_equal site.site_payload['site']['data']['members'], file_content
+      end
+
+      should 'auto load yml files' do
+        site = Site.new(Jekyll.configuration)
+        site.process
+
+        file_content = YAML.safe_load_file(File.join(source_dir, '_data', 'languages.yml'))
+
+        assert_equal site.data['languages'], file_content
+        assert_equal site.site_payload['site']['data']['languages'], file_content
+      end
+
+      should "load symlink files in unsafe mode" do
+        site = Site.new(Jekyll.configuration.merge({'safe' => false}))
+        site.process
+
+        file_content = YAML.safe_load_file(File.join(source_dir, '_data', 'products.yml'))
+
+        assert_equal site.data['products'], file_content
+        assert_equal site.site_payload['site']['data']['products'], file_content
+      end
+
+      should "not load symlink files in safe mode" do
+        site = Site.new(Jekyll.configuration.merge({'safe' => true}))
+        site.process
+
+        assert_nil site.data['products']
+        assert_nil site.site_payload['site']['data']['products']
+      end
+
+      should "load symlink directory in unsafe mode" do
+        site = Site.new(Jekyll.configuration.merge({'safe' => false, 'data_source' => File.join('symlink-test', '_data')}))
+        site.process
+
+        assert_not_nil site.data['products']
+        assert_not_nil site.data['languages']
+        assert_not_nil site.data['members']
+      end
+
+      should "not load symlink directory in safe mode" do
+        site = Site.new(Jekyll.configuration.merge({'safe' => true, 'data_source' => File.join('symlink-test', '_data')}))
+        site.process
+
+        assert_nil site.data['products']
+        assert_nil site.data['languages']
+        assert_nil site.data['members']
+      end
+    end
   end
 end