move ui functions out into their own autoload

All of the autoload shit needs to be cleaned up... break it out first
continue breaking up the autoload/nerdtree god module
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.
Merge branch 'master' of github.com:scrooloose/nerdtree