Add Support for the Twig Template Engine

This change will allow developers to use [Twig Templates](http://twig.sensiolabs.org/) as an alternative to the standard opencart template method. Templates with the `.twig` extension are rendered using the Twig   engine and are kept in the same `template` directory as standard templates. Backwards compatibility has been maintained as `tpl` and `twig` templates can be rendered together.

### Example - `common/home.tpl` as `common/home.twig`

```php
/* common/home.tpl */

<?php echo $header; ?>
<div class="container">
  <div class="row"><?php echo $column_left; ?>
    <?php if ($column_left && $column_right) { ?>
    <?php $class = 'col-sm-6'; ?>
    <?php } elseif ($column_left || $column_right) { ?>
    <?php $class = 'col-sm-9'; ?>
    <?php } else { ?>
    <?php $class = 'col-sm-12'; ?>
    <?php } ?>
    <div id="content" class="<?php echo $class; ?>"><?php echo $content_top; ?><?php echo $content_bottom; ?></div>
    <?php echo $column_right; ?></div>
</div>
<?php echo $footer; ?>

```

```jinja
{# common/home.twig #}

{% if column_left and column_right %}
{%  set class = 'col-sm-6' %}
{% elseif column_left or column_right %}
{%  set class = 'col-sm-9' %}
{% else %}
{%  set class = 'col-sm-12' %}
{% endif %}

{{ header }}
<div class="container">
  <div class="row">{{ column_left }}
    <div id="content" class="{{ class }}">{{ content_top }}{{ content_bottom }}</div>
    {{ column_right }}
  </div>
</div>
{{ footer }}
```

### Notes

**auto escaping** - Since rendered child templates are stored in variables and included in the main controller template, auto escaping had to be turned off. Otherwise `<html>` in the `$header` variable would be rendered as `&lt;html&gt;`

**`auto_reload`** - during development developers will want to enable `auto_reload` in the `Twig_Environment` object created in `system/library/twig.php`

```php
		$this->environment = new Twig_Environment($loader, array(
			'autoescape' => false,
			'cache' => DIR_CACHE . 'templates_c',
			'strict_variables' => false,
			'auto_reload' => true
		)); 
```

**composer** - I'd much rather have included Twig as a dependency using composer but I feel doing something like that would have a much larger impact on packaging opencart for both development and distribution. That's really not something I want to take on.