Added a cache for factories requested in Fixture.from static method.

A `ObjectFactory` operates like a singleton for each class when was created in `Fixture.from` static method. Example:

```java
Entity e1 = Fixture.from(Entity.class).gimme("valid");
Entity e2 = Fixture.from(Entity.class).gimme("valid");
```

This snippet call a `Fixture.from` method twice and create two `ObjectFactory` for two diferent fixtures from `Entity`. Two identical factory was created under the same `TemplateHolder` object provided by templates map in `Fixture` class. Even with no expansive computations theres no need to create a `ObjectFactory` in each call of `Fixture.from`. With this modification, a single `ObjectFactory` is created and two different fixtures for the `Entity` class.