Multiple external ports bound to a single internal port

Hi !

As I tested fig for my containers I realized that when using for ports declaration the following configuration :
```
  ports:
    - "80:80"
    - "5000:80"
```

Only the last binding was used.
It it due to the way the port_bindings dict is filled.
I changed that to allow this kind of configuration to work.
I think these are tabs, and they should be spaces (see the travis ci failures).

I might also suggest something like this, so you don't have to keep calling `split_port()` and indexing into the tuple:

```python

for port in options.get('ports') or []:
    internal, external = split_port(port)
    ports.setdefault(internal, []).append(external)
```

And a test to cover this case is probably also a good idea.