__construct() method and tidy-ups

# Changes

* Added optional quick __construct() method
* Capitalised class name - usual class naming convention
* Added spacing for readability
* Added `<?php` opening tag

# New usage

```php
$args = array(
	'isHTML'  => false,
	'to'      => 'to@email.com',
	'from'    => 'from@email.com',
	'replyTo' => 'replyto@email.com',
	'subject' => 'Subject',
	'message' => 'Message here',
	'charset' => 'utf-8',
	'errorMsg'=> 'Error!',
	'successMsg' => 'Thank you!'
);
$email = new Email($args);
$email->send();
```