decompose the giant NERD_tree.vim file

* Move the classes out into `plugin/nerdtree/<classname>`.
* Move the other functions out into `autoload/nerdtree.vim`.

Stuff still to do:

* extract out at least one view class from `autoload/nerdtree` -
  something like NERDTreeWindow
* figure out which functions in autoload/nerdtree should be scoped to
  the script instead of public