a barebone composer.json to install drupal 8.0 in a subfolder of a project-folder.The included autoload.php replaces the autoload.php from drupal, so all dependencies gets resolved.

autoload.php

<?php

/**
 * @file
 * Includes the autoloader created by Composer.
 *
 * @see composer.json
 * @see index.php
 * @see core/install.php
 * @see core/rebuild.php
 * @see core/modules/statistics/statistics.php
 *
 * This is a custom autoload-file, as we want, that drupal uses our vendor-lib,
 * not the one provided by drupal.
 */

return require __DIR__ . '/../vendor/autoload.php';

composer.json

{
  "name": "factorial/composer-in-drupal",
  "description": "using composer with drupal 8",
  "repositories": [
    {
      "type": "composer",
      "url": "https://packagist.drupal-composer.org/"
    }
  ],
  "require": {
    "composer/installers": "~1.0",
    "derhasi/composer-preserve-paths": "0.1.*",
    "wikimedia/composer-merge-plugin": "^1.3.0",
    "drupal/drupal": "8.*",
    "drupal/bootstrap": "8.x"
  },
  "extra": {
    "installer-paths": {
      "public": ["type:drupal-core"],
      "public/modules/contrib/{$name}": ["type:drupal-module"],
      "public/profiles/contrib/{$name}": ["type:drupal-profile"],
      "public/themes/contrib/{$name}": ["type:drupal-theme"],
      "public/themes/engines/{$name}": ["type:drupal-theme-engine"],
      "public/drush/commands/{$name}": ["type:drupal-drush"]
    },
    "preserve-paths": [
      "public/modules/contrib",
      "public/themes/contrib",
      "public/themes/engines",
      "public/themes/custom",
      "public/modules/custom",
      "public/libraries",
      "public/sites/default/settings.php",
      "public/sites/default/files"
    ],
    "merge-plugin": {
      "include": [
        "public/core/composer.json"
      ],
      "recurse": false,
      "replace": false,
      "merge-extra": false
    }
  },
  "minimum-stability": "dev",
  "prefer-stable": true,
  "scripts": {
    "post-install-cmd": [
      "cp autoload.php public/autoload.php"
    ]
  }
}