Allow overriding for ContextBuilder's loadClass

Allow overriding for **Class.forName(classPath)**

For PlayFramework 2 application, Class.forName only works for "production" but not in "dev" mode.

So with class inheritance I can define my own loadClass method

```
@Override
protected Class loadClass(String className) throws ClassNotFoundException {
        return Play.application().classloader().loadClass(className);
}
```