Fixed #12735, #14892 and #11542 -- Fixed support for the latest IPython...

Fixed #12735, #14892 and #11542 -- Fixed support for the latest IPython (development) version in the shell management command and added a hook to implement additional shell runners (by subclassing django.core.management.commands.shell.Command, extending the ``shells`` attribute and implement a method with the same name).

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14895 bcc190cf-cafb-0310-a4f2-bffc1f526a37