Provide an API module

Summary:
API replies in REST API tend to be:

• JSON representations of objects from a datasource, or a calculation
• Failures with relevant status code (400, 403, 404, 409, 500, etc.)

This module provides traits to consume values and get such API response.

It provides trait implementation for Diesel QueryResult, so you can do
something like this:

pub fn get_player(connection: DatabaseConnection, name: String) -> ApiJsonResponse<Player> {
    players
        .filter(username.eq(&name))
        .first::<Player>(&*connection)
        .into_json_response()
}

This will produce a JSON representation when the result is found,
a 404 error when no result is found, a 500 error if there is a database issue.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1663