Search stops served by a line

An user makes a GET request on `/buses/:busid` in order to obtain the list of stops served by that line. The response is shaped as a JSON array similar to the following:

``` javascript
[
  ...,
  {
    stopname: "stop name",
    stopnumber: 9999,
    position: {
      lat: 99.9999,
      lon: 99.9999
    }
  },
  ...
]
```
