Using rmccue/requests to avoid depending on ext-curl

Replaced unirest with rmccue/requests, to avoid depending on ext-curl. This version runs on a freshly installed php with no external dependencies.

I also modified composer.json to allow the usage of composer's native autoloading.
Here's an example client code:

```php
<?php
require 'vendor/autoload.php';

$mango = new Mango\Mango([
    "api_key" => "..."
]);

$customer = $mango->Customers->create(["email" => 'fonso@gmail.com']);
$mango->Customers->delete($customer->uid);
```