Fixed #10771 -- added support for using the transaction management functions...

Fixed #10771 -- added support for using the transaction management functions as context managers in Python 2.5 and above.  Thanks to Jacob for help with the docs.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14288 bcc190cf-cafb-0310-a4f2-bffc1f526a37