Set tmpRoot on tree if tmpRoot provided to builder

I know this needs tests, but I wanted to start the discussion. This is for #202.
My node sucks :)

I think where I'm using it below in `readAndReturnNodeFor` I'm deep inside some promise and didn't have easy access to the instance. I can figure that out though and fix it. I was mostly hacking through this stuff as fast as i could at a meetup, I intended to cleanup and write tests later.