As long as composer support in CMS is "not there yet", you need to get around somehow.

Say you want to use the (awesome) markdown library, you need a way to get it in.

1. Use a container extension with a private namespace
2. Create composer.json
3. Include the autoloader for both contexts
4. profit

Create a file composer.json in your container extension in Resources/Private (you can use composer init):

{
    "name": "cedricziel/packagename",
    "config": {
      "vendor-dir": "Libraries"
    },
    "require": {
        "michelf/php-markdown": "~1.4"
    },
    "authors": [
        {
            "name": "Cedric Ziel",
            "email": "cedric @t cedric-ziel.com"
        }
    ]
}

Note the vendor-dir directive. This allows you to determine where composer will put the libs and the class loader you have to include.

I cant explain the exact sematics very well, but you will want to have both. Otherwise your classes wont load in a cached context.

ext_tables.php

<?php
if (!defined('TYPO3_MODE')) {
	die('Access denied.');
}

$composerAutoloadFile = \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::extPath($_EXTKEY)
	. 'Resources/Private/Libraries/autoload.php';

require_once($composerAutoloadFile);

ext_localconf.php

<?php
if (!defined('TYPO3_MODE')) {
	die('Access denied.');
}

$composerAutoloadFile = \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::extPath($_EXTKEY)
	. 'Resources/Private/Libraries/autoload.php';

require_once($composerAutoloadFile);

This is a workaround - let me get this straight. Problems could arise if you do that overly often with many different libraries and end up with different loadable classes. I therefore propose you creat one such container extension per project.