Emit the 'runner:screenshot' event

[adenflorian]

This library doesn't seem to emit the runner:screenshot event when it takes a screenshot.

Create a custom reporter that listens for runner:screenshot

    this.on('runner:screenshot', function (payload) {
        console.log('screenshot taken');
    });

Then have a test call browser.saveDocumentScreenshot(fileName);

'screenshot taken' is never logged.

[IgorVodka]

Made a pull request. #91