Invoke autoload functions instead of custom commands

Pull request #31: https://github.com/xolox/vim-session/pull/31

Ingo's explanation (which I completely agree with):

  Some people using the :Session... alternatives may :delcommand the
  original commands; in that case, the invocations by the script would
  fail. Prevent this error by invoking the autoload functions directly;
  this also obsoletes the need for fnameescape().