Update closure util

This updates the `closure-util` package to a version that allows configuration of the Closure Compiler and Library versions.  The [readme](https://github.com/openlayers/closure-util/blob/master/readme.md#configuration) describes configuration options.

As an example, if you wanted to test an [alternate revision](https://code.google.com/p/closure-library/source/browse/?r=6fb09f438660ca9b909d02d69dd5b45564c1c8ed) of the Closure Library, the following `closure-util.json` file could be added (at the root of the ol3 repo):

```json
{
  "library_url": "https://closure-library.googlecode.com/archive/ab89cf45c216615d73a2f5dea720afb9d3415d1f.zip"
}
```

After choosing an alternate version of the library, run the following:
```
npm install
```

(The archive URL for the library can be found using the "Download zip" link on the [repository browser](https://code.google.com/p/closure-library/source/browse/).)

The Closure Compiler URL can be set in a similar way using the `compiler_url` key.  Though the downloaded compiler is not currently used by the build tools.

As before, you start the Node based server with:
```
npm start
```

After the server starts, you can browse around (http://localhost:3000/) and run tests (http://localhost:3000/test).  The example index currently needs to be rebuilt with `build.py examples` (though this can eventually become a Node task as well). 

The log level is currently set to `info`.  If you want more verbose logging, this can be set via an environment variable (or a key in the `closure-util.json` file).  E.g.
```
closure_log_level=verbose npm start
```
(Note that environment variables for the `closure-util` package are prefixed with `closure_`.)