continue breaking down the epic autoload module

Add 2 new classes and move code into them from autoload:

* NERDTree. Each nerdtree buffer now has a NERDTree object that holds
  the root node and will old other util functions
* UI. Each NERDTree object holds a UI object which is responsible for
  rendering, getting the current node, etc

Still a fair few methods to sort through in autoload (many of which will
end up in the above classes) - need sleep though.

lib/nerdtree/opener.vim

@@ -262,7 +262,7 @@ function! s:Opener._openDirectory(node)
         call self._gotoTargetWin()
         if empty(self._where)
             call a:node.makeRoot()
-            call nerdtree#renderView()
+            call b:NERDTree.render()
             call a:node.putCursorHere(0, 0)
         elseif self._where == 't'
             call g:NERDTreeCreator.CreatePrimary(a:node.path.str())