Add Batch queries method

Add the ability to batch multiple queries into 1 request to analytics and return results as an array.

Usage:

```php
$requests = array(
        array(
                'key' => 'req1',
                'id' => 'ga:xxxxxx',
                'startDate' => '2015-15-09',
                'endDate' => '2015-15-09',
                'metrics' => 'ga:pageViews',
                'others' => array(
                    'filters' => 'ga:pagePath==/'
                )
            ),
        array(
                'key' => 'req2',
                'id' => 'ga:xxxxxx',
                'startDate' => '2015-15-09',
                'endDate' => '2015-15-09',
                'metrics' => 'ga:goalCompletionsAll'
            )
        );

        $result = \Analytics::batchQueries($requests);
        
        $req1 =  $result['response-req1'];
        $req2 = $result['response-req2'];
```