add ability to supply user agent to analytics

Without sending a `User-Agent` header browse, platform, and mobile capabilities are not computed ([docs](https://developers.google.com/analytics/devguides/collection/protocol/v1/reference#transport)). This adds the ability to set a user-agent on the UA object for all the tracks/requests to use. 

Example:

````php
$ua = new \UniversalAnalytics\UA(array(
    'v' => 1,
    'tid' => 'UX-XXXX-XXX',
    'cid' => 555,
), $_SERVER['HTTP_USER_AGENT']);

// do everything else the same
````