Add oldest query parameter for Olympians endpoint

As a user,

I can submit a request to the endpoint /api/v1/olympians with a query parameter of age=oldest and receive the oldest Olympian in the system, formatted identically to the normal index.

Sample response:

``` JSON
{
"olympians": [
{
"name": "Julie Brougham",
"team": "New Zealand",
"age": 62,
"sport": "Equestrianism"
"total_medals_won": 0
}
]
}
```