Add stop/shutdown task to Router

Not sure if a mix task would be of any use here, so I just implemented the stop function in the router.

#66 

Thoughts?

```text
$ iex -S mix phoenix.start
Erlang/OTP 17 [erts-6.0] [source] [64-bit] [smp:8:8] [async-threads:10] [hipe]
[kernel-poll:false]

...

Running Elixir.PhoenixDev.Router with Cowboy on port [dispatch: [_: [{:_,
Plug.Adapters.Cowboy.Handler, {PhoenixDev.Router, []}}]], port: 4000, ssl:
false, consider_all_requests_local: true]
Interactive Elixir (0.13.2) - press Ctrl+C to exit (type h() ENTER for help)

iex(1)> PhoenixDev.Router.stop
Elixir.PhoenixDev.Router has been stopped
:ok
iex(2)>
```