return $this from setters to allow for method chaining.

Allow for method chaining, and more of a fluent interface when writing `Query` classes etc.

for example:
```php
$query = $this->get('widop_google_analytics.query')
    ->setIds('ga:XXXXXXXX')
    ->setStartDate(new \DateTime('2012-01-01'))
    ->setEndDate(new \DateTime());
```

Perhaps this is just my personal preference, but feel free to merge :thumbsup: