fix(groups): actually add groups support for capture

[macobo]

I added groups feature in #26 but didn't add all the functionality I advertised.

Tested via:

<?php

require_once __DIR__ . '/vendor/autoload.php';

use PostHog\PostHog;

PostHog::init("phc_DZgJcBXRIkZvPpwiwrKSyUqTVtz8qhiFzaED6ldjcT9", array('host' => 'http://localhost:8000', 'ssl' => false));

PostHog::capture(array(
  'distinctId' => 'user:123',
  'event' => 'movie played',
  '$groups' => array("organization" => "id:5")
));

PostHog::groupIdentify(array(
  'groupType' => 'organization',
  'groupKey' => 'id:5',
));

PostHog::flush();

?>

In posthog:

[neilkakkar]

D'oh, makes sense!

[tiina303]

any reason this isn't merged?