Added a generic "button" class to the admin styles to allow applying the...

Added a generic "button" class to the admin styles to allow applying the standard admin button styles to arbitrary elements in the future. Should have no effect on existing code.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9077 bcc190cf-cafb-0310-a4f2-bffc1f526a37