Implement views_handler_argument->title() for argument titles.

When building a view that allows for title overrides based on argument values, date_views doesn't implement the title() function in the views handler. As such, I only ever see the raw argument out of the URL as the page title. It should wash this through the display date format for the view the same as is done for summary links. This can be done by adding a title() function to date_views_argument_handler_simple.inc that returns the argument value as formatted by date_make_date.

I did a simple implementation, though this probably isn't as robust as it could be. This patch is against Alpha 4, as the current dev was broken when I tried it.