memory leak in concurrent task interface

after $client->run_tasks() the task object is never freed, causing a
memory leak.

run the included sample client and watch the process size. On
my system is grows roughly by 1MB every second.

example:
use Gearman::XS qw(:constants);
use Gearman::XS::Client;

my $client = new Gearman::XS::Client;
$client->add_server();

while (1) {
 $client->add_task("reverse", "blubb");
 $client->run_tasks();
}