Added support for Dockerfile's EXPOSE command

In the plugin's configuration element in the pom, you can now specify
which ports you would like to expose. For example:

```xml
  <plugin>
    <groupId>com.spotify</groupId>
    <artifactId>spotify-docker-maven-plugin</artifactId>
    ...
    <configuration>
      ...
      <exposes>
        <expose>1234</expose>
      </exposes>
      ...
    </configuration>
  ...
  </plugin>
```

Note that you still need to publish that port when running the container
to make it available to the host machine (e.g. docker run -P=true ...)