Android: Prepare for Espresso support

This PR aims at making it possible to also use the Espresso framework in step definitions.

I don't find Espresso officially published through Maven, which Cucumber-JVM uses both for CI builds and deployment. Therefore Espresso is not introduced as dependency, but the PR makes it really easy to write a custom Espresso runner locally. Using the CucumberInstrumentation works exactly as before.

A local runner like this, has been successfully tested in the [Cukeulator-test](https://github.com/cucumber/cucumber-jvm/tree/master/examples/android/cukeulator-test) example:
```
public class EspressoInstrumentation extends GoogleInstrumentationTestRunner {
    private CucumberInstrumentationHelper helper = new CucumberInstrumentationHelper(this);

    @Override
    public void onCreate(Bundle arguments) {
        helper.onCreate(arguments);
        super.onCreate(arguments);
    }

    @Override
    public void onStart() {
        helper.onStart();
    }
}

```

Fixes #662.

Notes on the what I found out about the current status of Maven and Espresso:
In [Quality-Tools-for-Android](https://github.com/stephanenicolas/Quality-Tools-for-Android) the Espresso jar is checked in as a local Maven [repo](https://github.com/stephanenicolas/Quality-Tools-for-Android/tree/master/android-sample-espresso-tests/repo/com/google/android/android-espresso).
A [Gradle port](https://github.com/JakeWharton/double-espresso/) of Espresso is available through [Maven Central](http://search.maven.org/#search|ga|1|espresso)