Allow custom injector creation

I recently started using [Netflix Governator](https://github.com/Netflix/governator), and have been trying to get it working with Jukito, but have had some issues.

This PR makes it possible to use governator by allowing `JukitoRunner` to be extended and `createInjector` to be overridden. For example, I've overridden it like this:

```
@Override
protected Injector createInjector(TestModule testModule)
{
    LifecycleInjector lifecycleInjector = LifecycleInjector.builder()
                            .withBootstrapModule(new MyCustomBootstrapModule())
                            .withAdditionalModules(testModule)
                            .build();

    Injector injector = lifecycleInjector.createInjector();
    lifeCycleManager = injector.getInstance(LifecycleManager.class); // start/stop this later
    return injector;
}
```

I know that `JukitoRunner` has a constructor that takes an injector, but if this constructor is used, then `ensureInjector` will not run.