Add medalists endpoint for a specific event

As a user,

I can request a list of the medalists for a specific event by submitting a GET request to the endpoint `/api/v1/events/:id/medalists`

I will receive the following:

``` JSON
{
"event": "Badminton Mixed Doubles",
"medalists": [
{
"name": "Tontowi Ahmad",
"team": "Indonesia-1",
"age": 29,
"medal": "Gold"
},
{
"name": "Chan Peng Soon",
"team": "Malaysia",
"age": 28,
"medal": "Silver"
}
]
}
```