suport method chaining

[PauloPhagula]

Issue Type

• Task

Summary

The way this is used normally involves calling many methods on the tracker one after another, so add method chaining to make it a bit more easier to use and lessen typing a bit.

So for example this

$tracker->trackTransaction($cid, $fake_payment_id, "Biscate", $subscription_cost_in_usd, 0, 0, "USD");
$tracker->trackTransactionItem($cid, $fake_payment_id, $msisdn, $subscription_cost_in_usd, 1, NULL, "worker-registration", "USD");

could be written as

$tracker
  ->trackTransaction($cid, $fake_payment_id, "Biscate", $subscription_cost_in_usd, 0, 0, "USD")
  ->trackTransactionItem($cid, $fake_payment_id, $msisdn, $subscription_cost_in_usd, 1, NULL, "worker-registration", "USD");