[GH-365] DDC-1860 - Composer arbitrary for CLI and composer/autoload.php in different paths

This issue is created automatically through a Github pull request on behalf of Ocramius:

Url: https://github.com/doctrine/doctrine2/pull/365

Message:

DDC-1860 - Composer became a requirement to run CLI some time ago. This patch:

1. Makes composer optional if the user defines his own autoloaders in `cli-config.php`
2. Makes the CLI look for `vendor/composer.php` in various parent paths of the `bin` directory.