Add engine factory

[eduardosoliv]

This makes more convenient to use the string template.

Instead of:

$engine = new StringTemplate\Engine();
$enginer->render(...);

Can be used:

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.

Note: I didn't put any license on header of new files, because on code base there are 2 types of headers, so didn't know which one to use.

[nicmart]

I don't see your example as a valid reason to use a factory. Factory is a creational pattern, and should be used wisely. Direct method invocation is not a good reason to use a factory, in my opinion. Also note that in php >= 5.4 you can use direct method invocation after instantiation:

(new Engine)->render(...);

Even if I discourage it, if you need static access to Engine methods I think it is better to implement an adaptor on the client side.