Allow weaved byte code to be passed to Robolectric.

This simple change allows to pass transformed/weaved byte code to Robolectric via a custom RobolectricTestRunner : 

```java
public class CustomTestRunner extends RobolectricTestRunner {

  public CustomTestRunner(Class<?> testClass) throws InitializationError {
    super(testClass);
  }

  @Override protected ClassLoader createRobolectricClassLoader(Setup setup, SdkConfig sdkConfig) {
    URL[] urls = getJarResolver().getLocalArtifactUrls(sdkConfig.getSdkClasspathDependencies());
    return new AsmInstrumentingClassLoader(setup, urls) {
      @Override protected byte[] getByteCode(String className) throws ClassNotFoundException {
       // you can weave bytecode here and pass it to Robolectric
       // and/or call super..
      }
    };
  }
}
```

I need this mechanism for https://github.com/stephanenicolas/injectview/

I would love if it could be part of 2.4.0 as it doesn't change the API. 

Thx in advance.