On session restore, maintain cwd after loading NERDTree

From Matthew:

  There's a NERDTree option which will change the working directory to
  its root directory when opened. Our sessions should keep their working
  directory despite this option.

  This change records the working directory before loading NERDTree,
  load it, then cd to this saved directory.

Pull request #37: https://github.com/xolox/vim-session/pull/37