Add engine factory

This makes more convenient to use the string template.

Instead of:
```php
$engine = new StringTemplate\Engine();
$enginer->render(...);
```

Can be used:
```php
EngineFactory::create()->render(...);
```

If you see other libs that are similar:
https://github.com/jalet/util-sprintf-php

The way of use is really simple:
```
Sprintf::f("My cat is %color%.", array('color' => 'brown'));
```

I prefer StringTemplate over that one, but I would prefer a more convenient way to use it, at least a factory.