Add task for parsing examples

This adds a Node based task that does the equivalent of `exampleparser.py`.  The Python script is left in place, so old workflows still work.  With the addition of this task, a lightweight dev environment is available with:

    npm install
    npm start

The `npm install` step downloads Closure Library, Closure Compiler, and other dev dependencies.  A post-install step creates the `example-list.js` script used by the example index.

The `npm start` step starts a development server that provides a static file server and a script loader for examples and tests (the script loader provides ol3 and Closure dependencies).

The `exampleparser.py` script does a few things that are no longer necessary (e.g. generating the unused `example-list.xml` feed).  With or without that extra work, the new task is faster:

```
$ time ./build.py examples/example-list.js

real	0m2.933s
```

```
$ time node tasks/parse-examples.js 

real	0m0.148s
```