Adds excerpt to posts

test/source/_posts/2013-01-02-post-excerpt.markdown

@@ -0,0 +1,14 @@
+---
+layout: ~
+title:  Post Excerpt
+---
+
+First paragraph with [link ref][link].
+
+Second paragraph
+
+---
+
+Third paragraph
+
+[link]: http://www.jekyllrb.com/

test/test_generated_site.rb

@@ -14,7 +14,7 @@ class TestGeneratedSite < Test::Unit::TestCase
     end
 
     should "ensure post count is as expected" do
-      assert_equal 30, @site.posts.size
+      assert_equal 31, @site.posts.size
     end
 
     should "insert site.posts into the index" do

test/test_post.rb

@@ -252,6 +252,52 @@ class TestPost < Test::Unit::TestCase
 
         assert_equal "<h1>{{ page.title }}</h1>\n<p>Best <strong>post</strong> ever</p>", @post.content
       end
+
+      context "#excerpt" do
+        setup do
+          file = "2013-01-02-post-excerpt.markdown"
+          @post.process(file)
+          @post.read_yaml(@source, file)
+          @post.transform
+        end
+
+        should "return first paragraph by default" do
+          assert @post.excerpt.include?("First paragraph"), "contains first paragraph"
+          assert !@post.excerpt.include?("Second paragraph"), "does not contains second paragraph"
+          assert !@post.excerpt.include?("Third paragraph"), "does not contains third paragraph"
+        end
+
+        should "correctly resolve link references" do
+          assert @post.excerpt.include?("www.jekyllrb.com"), "contains referenced link URL"
+        end
+
+        should "return rendered HTML" do
+          assert_equal "<p>First paragraph with <a href='http://www.jekyllrb.com/'>link ref</a>.</p>",
+                       @post.excerpt
+        end
+
+        context "with excerpt_separator setting" do
+          setup do
+            file = "2013-01-02-post-excerpt.markdown"
+
+            @post.site.config['excerpt_separator'] = "\n---\n"
+
+            @post.process(file)
+            @post.read_yaml(@source, file)
+            @post.transform
+          end
+
+          should "respect given separator" do
+            assert @post.excerpt.include?("First paragraph"), "contains first paragraph"
+            assert @post.excerpt.include?("Second paragraph"), "contains second paragraph"
+            assert !@post.excerpt.include?("Third paragraph"), "does not contains third paragraph"
+          end
+
+          should "replace separator with new-lines" do
+            assert !@post.excerpt.include?("---"), "does not contains separator"
+          end
+        end
+      end
     end
 
     context "when in a site" do