Place the nerd tree below the BufExplorer

Now, I have been put NERDTree right of the screen and disable it when input `:edit <directory>` to use NERDTree to open it. It will still use netrw。

```
let g:NERDTreeWinPos = "right"
let NERDTreeHijackNetrw=0
```

Anyway, I want to put NERDTree below BufExplorer and the TagList right.

But I did not find out how to make it,  `NERD_tree.vim` has not appropriate setting for this.
