title: capture screenshots only on failure with allure reporter
body: <p>I fallowed tutorial on <a href="https://github.com/allure-framework/allure-jasmine" rel="nofollow noreferrer">https://github.com/allure-framework/allure-jasmine</a>
  I get a screenshot every spec I run even if pass. 
  I want to achieve, that screenshots will be captured only when test fails.</p>
  
  <p>adding screenshot to test(this is how my conf.js looks like)</p>
  
  <pre><code>  onPrepare: function () {
      var AllureReporter = require('jasmine-allure-reporter');
      jasmine.getEnv().addReporter(new AllureReporter());
      jasmine.getEnv().afterEach(function(done){
        browser.takeScreenshot().then(function (png) {
          allure.createAttachment('Screenshot', function () {
            return new Buffer(png, 'base64')
          }, 'image/png')();
          done();
        })
      });
    }
  </code></pre>