Google Analytics API Integration.

Adds Google Analytics API Integration so you can use GA data in your reports.  

Google's Core Reporting documentation is helpful in getting started - https://developers.google.com/analytics/devguides/reporting/core/v3/

Usage
---------------
First, set the `ga_api` config parameters in `config/config.php`

```php
return array(
  //...
  'ga_api'=>array(
    'applicationName'=>'PHP Reports',
    'clientId'=>'abcd1234',
    'clientSecret'=>'1234abcd',
    'redirectUri'=>'http://example.com'
  ),
  //...
);
```

You need to add a GA Authentication link somewhere in your `templates/local/html/page.twig` template (create this file if it doesn't exist).  This example puts a link in the top nav.

```html+jinja
{% extends "default/html/page.twig" %}

{% block navbar %}
{{ parent() }}
<a class="navbar-text" href="{{base}}/ga_authenticate?redirect={{(path ~ request.url ~ '/?' ~ querystring) | url_encode}}">
  {% if session.ga_token %}<i class="glyphicon glyphicon-ok"></i> {% endif %}
  GA Authenticate
</a>
{% endblock %}
```

Then in a PHP report:
```php
<?php
// My GA Report

// Make sure the user is authenticated
if(!isset($_SESSION['ga_token'])) throw new Exception("Use the GA Authenticate link in the top nav before running this report");

// Reference to the google service class
global $ga_service;

// Core reporting API call
$response = $ga_service->ga_data->get('ga:1234','2014-01-01','2014-02-01','ga:users',array(
  'dimensions': 'ga:language'
));

//...
```