add document:exists

[y-abs]

What does this PR do ?

This PR implements the document:exists method with its unit tests.

How should this be manually tested?

Clone this branch and run unit tests
./gradlew test

When it succeed, compile it

./gradlew jar

Initiate another java project by adding the compiled SDK as a dependency.

Then, run Kuzzle, create an index nyc-open-data and a yellow-taxi collection in the admin console.
Finally, run this code

import io.kuzzle.sdk.*;
import io.kuzzle.sdk.Options.KuzzleOptions;
import io.kuzzle.sdk.Options.Protocol.WebSocketOptions;
import io.kuzzle.sdk.Protocol.WebSocket;

import java.util.concurrent.ConcurrentHashMap;

public class existsDocument {
    private static Kuzzle kuzzle;

    public static void main(String[] args) {
        WebSocketOptions opts = new WebSocketOptions();
        opts.setAutoReconnect(true).setConnectionTimeout(42000);

        try {
            WebSocket ws = new WebSocket("localhost", opts);

            kuzzle = new Kuzzle(ws, (KuzzleOptions) null);

            kuzzle.connect();
            kuzzle.getDocumentController().exists("nyc-open-data", "yellow-taxi", "some-id")
            .get();
        }  catch (Exception e) {
            e.printStackTrace();
        }

        kuzzle.disconnect();
    }
};

[scottinet]

wrong return type