Respect 'Accept' header to serve GeoJSON as application/vnd.geo+json
Only return GeoJSON for /api/tram_stops if client accepts it

A custom JSON is returned if the client accepts application/json, for
all other cases HTTP 406 Not Acceptable is returned.
Merge pull request #11 from metaodi/geojson-content-type

Respect 'Accept' header to serve GeoJSON as application/vnd.geo+json