Added restart_delay option to Poolboy

In order to don't crash the whole pool when a start/restart fails (probably because the remote connection is down) we added the option "restart_delay". This has been implemented trying to be as least intrusive as possible so, if omitted, the pool would keep working as it currently is, crashing if the start_link process fail. In case overflow is also enabled overflow worker would be reused as a regular worker if the pool is about to restart a delayed worker. The atom "dead_endpoint" will be returned, allowing the process requesting a worker to try a different pool.

A couple of Eunit tests have been added to poolboy_tests as well as new worker, poolboy_test_worker_delay, that would fail in its startup function according to a counter.