Use monitors instead of links and trap_exit

We encountered a problem similar to the one stated in http://stackoverflow.com/questions/19354937/strange-error-message-when-stopping-app-using-lager-and-poolboy. The problem can be described as follows: If poolboy is used in a supervisor tree, shutting down the tree might cause error messages similar to this one:

    =SUPERVISOR REPORT==== 22-Oct-2013::17:51:45 ===
         Supervisor: {<0.252.0>,poolboy_sup}
         Context:    shutdown_error
         Reason:     shutdown
         Offender:   [{nb_children,1},
                      {name,our_worker},
                      {mfargs,
                          {our_worker,start_link,
                              [[{name,{local,our_worker_pool_sup}},
                                {size,5},
                                {overflow,5},
                                {worker_module,our_worker}]]}},
                      {restart_type,temporary},
                      {shutdown,5000},
                      {child_type,worker}]

This means that a worker is shutdown by poolboy without using the poolboy supervisor. Such behaviour can occur if the poolboy `gen_server` shuts the workers down through its links to the workers. The attached commits fixes this by substituting the links between the poolboy process and the workers with monitors.