Provide a way to load JSON data for the server

The idea of this PR is a way to get JSON into the browser. This might be interesting when designing views for data that is loaded with Ajax, e.g some movies collection.

By putting a file ./data/movies.json with content:

    [{ "title": "Midnight in Paris" }]
  
it is possible to load the JSON from the browser with e.g.:

    $.get("/movies.json", function(data) { console.log(JSON.parse(data)); })

results into

    [ { title: 'Midnight in Paris' }]

Underlying this is Sinatra reading all .json files from the ./data directory.

What do you think?