Added a new ''modified:' metadata tag to be able to specify the

publication time and date and the last modified time and date
independently.

This makes it possible to access the last updated date with {{ article.locale_modified }} in templates.
Additionally, an already delivered feed entry can be corrected by changing the modified date and time, as it is used for atom:update
/ rss pubDate field now.

pelican/tests/content/article_with_markdown_and_footnote.md

@@ -1,8 +1,9 @@
 Title: Article with markdown containing footnotes
 Date: 2012-10-31
+Modified: 2012-11-01
 Summary: Summary with **inline** markup *should* be supported.
 
 This is some content[^1] with some footnotes[^footnote]
 
 [^1]: Numbered footnote
-[^footnote]: Named footnote
+[^footnote]: Named footnote

pelican/tests/content/article_with_markdown_and_nonascii_summary.md

@@ -1,6 +1,7 @@
 Title: マックOS X 10.8でパイソンとVirtualenvをインストールと設定
 Slug: python-virtualenv-on-mac-osx-mountain-lion-10.8
 Date: 2012-12-20
+Modified: 2012-12-22
 Tags: パイソン, マック
 Category: 指導書
 Summary: パイソンとVirtualenvをまっくでインストールする方法について明確に説明します。

pelican/tests/content/article_with_md_extension.md

@@ -2,6 +2,7 @@ Title: Test md File
 Category: test
 Tags: foo, bar, foobar
 Date: 2010-12-02 10:14
+Modified: 2010-12-02 10:20
 Summary: I have a lot to test
 
 Test Markdown File Header

pelican/tests/content/article_with_metadata.rst

@@ -4,6 +4,7 @@ This is a super article !
 
 :tags: foo, bar, foobar
 :date: 2010-12-02 10:14
+:modified: 2010-12-02 10:20
 :category: yeah
 :author: Alexis Métaireau
 :summary: