FIX clean shutdown in supervision tree

This is a really minimal fix to solve error reports like this:
```
(my_node@pro13)1> application:stop(my_app).
ok
(my_node@pro13)2> 17:12:53.342 [error] Supervisor {<0.113.0>,poolboy_sup} had child my_child started with my_child:start_link([...]) at undefined exit with reason shutdown in context shutdown_error
17:12:53.346 [info] Application my_app exited with reason: stopped
```
This fixes this particular issue and nothing else, which is really well described in answer number 2 here: http://stackoverflow.com/questions/19354937/strange-error-message-when-stopping-app-using-lager-and-poolboy

The proper fix is having poolboy.erl not linking to the workers but monitoring them, as attempted in #37.