add client libraries for New Relic Browser

See: https://github.com/GigaOM/gigaom/issues/5525
If we _really_ wanted to move this JS elsewhere, we could do something like the following.  I'm not pushing for that, mind you.

```php
<script>
  <?php
  echo file_get_contents( __DIR__ . '/js/thing.js' );
  ?>;NREUM.info=<?php echo json_encode( $this->settings ); ?>
</script>
```