Pre bootstrap hook

I added a pre bootstrap hook that allows to do something like https://github.com/ArcBees/GWTP/issues/103. Also to simplify the generator a default bootstrapper has been added that is used if no custom implementation is found.

Example:
```java
@PreBootstrap
public class PreBootstrapperImpl implements PreBootstrapper {
    @Override
    public void onPreBootstrap() {
        GWT.setUncaughtExceptionHandler(new GWT.UncaughtExceptionHandler() {
            @Override
            public void onUncaughtException(final Throwable e) {
                Window.alert(e.getMessage());
            }
        });
    }
}
```

The generated ApplicationController if a pre bootstrapper is found:
```java
public class ApplicationControllerImpl implements ApplicationController {
  @Override
  public void init() {
    new PreBootstrapperImpl().onPreBootstrap();
    
    Scheduler.get().scheduleDeferred(new ScheduledCommand() {
      @Override
      public void execute() {
        DelayedBindRegistry.bind(com.gwtplatform.mvp.client.ClientGinjector.SINGLETON);
        
        com.gwtplatform.mvp.client.ClientGinjector.SINGLETON.getBootstrapperImpl().onBootstrap();
      }
    });
  }
}
```


The generated ApplicationController if no pre bootstrapper is found:
```java
public class ApplicationControllerImpl implements ApplicationController {
  @Override
  public void init() {
    DelayedBindRegistry.bind(com.gwtplatform.mvp.client.ClientGinjector.SINGLETON);
    
    com.gwtplatform.mvp.client.ClientGinjector.SINGLETON.getBootstrapperImpl().onBootstrap();
  }
}
```