I found an error when I tried to use [git_menu.vim](g:NERDTreeMenuController):
```
Error detected while processing function nerdtree#invokeKeyMap..91..90..<SNR>52_showMenu..65..82:
line 2:
E121: Undefined variable: s:MenuController
E15: Invalid expression: s:MenuController.New(self.children)
line 3:
E121: Undefined variable: mc
```
I searched the line at which it happened
then, I found that `s:MenuController` is not defined in `menu_item.vim` but in menu_controller.vim .
So I corrected the name to refer to MenuController.
On some new versions of vim these files were being loaded before the
main NERD_tree.vim which was causing errors as dependencies werent
loaded in time. Move the classes into lib - so vim wont try to load them
until we tell it
