Log the port that Cowboy starts on

Before it wasn't clear:

```
hex_web[master]% foreman start
12:00:26 web.1  | started with pid 59220
12:00:28 web.1  | [Porcelain]: goon executable not found
12:00:28 web.1  | [Porcelain]: falling back to the basic driver
12:00:28 web.1  |
```

After:

```
hex_web[add_log_for_port]% foreman start
11:59:37 web.1  | started with pid 58891
11:59:38 web.1  | [Porcelain]: goon executable not found
11:59:38 web.1  | [Porcelain]: falling back to the basic driver
11:59:38 web.1  |
11:59:38 web.1  | 2014-10-11 11:59:38.617 [info] Starting Cowboy on port 5000
```