Use the closure-util package

This is an initial move to use Node based packages for our development tools.  The `closure-util` package provides what could be a replacement for Plovr.  As it is integrated in these changes, it just provides an alternative development server.

To use it, first get all the dependencies:

    npm install

Then start the development server:

    npm start

At this point, you should be able to browse the repo at http://localhost:3000/.  In an effort to keep these changes to a minimum, I didn't change how the example index is built.  So to view the `example/index.html` page you still have to `build.py examples/example-list.js` first.

Next, I'd like to change how we build our examples.  But I wanted to keep this easy to review.  Note that it doesn't currently change anything about how we build `ol.js`, lint, etc.