Corrections to guides for 4 Sep release

Items repaired:
- Pin_discussion.png file in wring directory
- Discussion_filters.png image in discussion_students unreadable,
replaced
- updated  release date in Research guide change log
- updated  release date in Course Author’s guide change log
- updated  “History” dates for fields added to research guide

docs/en_us/course_authors/source/change_log.rst

@@ -12,7 +12,7 @@ September, 2014
 
    * - Date
      - Change
-   * - 09/02/14
+   * - 09/04/14
      - Updated the :ref:`Discussions` and :ref:`Discussions for Students and
        Staff` chapters to include information about choosing the type of post
        and to reflect changes in the user interface.

docs/en_us/data/source/internal_data_formats/change_log.rst

@@ -11,7 +11,7 @@ Change Log
 
    * - Date
      - Change
-   * - 09/02/14
+   * - 09/04/14
      - Updated the :ref:`Discussion Forums Data` chapter to include the
        ``thread_type`` field for CommentThreads and the ``endorsement`` field
        for Comments.

docs/en_us/data/source/internal_data_formats/discussion_data.rst

@@ -333,7 +333,7 @@ thread_type
 --------------------
   Identifies the type of post as a "question" or "discussion".  
 
-  **History**: Added 2 Sep 2014.
+  **History**: Added 4 Sep 2014.
 
 ********************
 Comment Fields
@@ -378,7 +378,7 @@ endorsement
   response to a post was endorsed and the numeric user ID (from
   ``auth_user.id``) of the person who endorsed it.
 
-  **History**: Added 2 Sep 2014.
+  **History**: Added 4 Sep 2014.
 
 --------------------
 comment_thread_id