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 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.