Add \Handlebars\SafeString class

Line helpers can return a result wrapped into ```Handlebars\safeString``` class to prevent auto escaping.

Example of usage:
```php
$engine = new \Handlebars\Handlebars();

$engine->addHelper('copyrigth', function() {
    return new \Handlebars\SafeString('Copyright 2014 <a href="example.com">Example dot com</a>');
});
```