Introduce custom PSR-4 autoload if not using composer

[julienbourdeau]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Related Issue	Fix #...
Need Doc update	yes

What was changed

Adding a custom autoload. If you don't use composer, you can require the new autoload.php.

Example:

<?php

require 'lib/algolia/autoload.php'; # example path

$client  = Client::create();

Why it was changed

Some older projects (WordPress, Drupal 7, and such) aren't using composer for autoloading). For v1, they rely on a file that require all the existing file: algoliasearch.php.

This new file sets a custom autoloader instead of requiring all the things.