Add the NERDTreeCreator class.
Stick all functions related to creating a primary/secondary/mirror
nerdtree in there. We may break this down further in the future, but
this is a good starting point.
Make some of the interface binding functions in autoload/nerdtree
public. This is needed since we are accessing some of them from
NERDTreeCreator. Should be temporary until we get some kind of proper
interface binding system set up.
* 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
