code: function sendScreenshot (name, runId, metaData, properties, compareSettings, png) {
  	return _callServer('post', 'runs/' + runId + '/screenshots', null, {
  		meta: JSON.stringify(metaData),
  		properties: JSON.stringify(properties),
  		compareSettings: JSON.stringify(compareSettings),
  		screenshotName: name,
  		file: {
  			value: new Buffer(png, 'base64'),
  			options: {
  				filename: 'file.png',
  				contentType: 'image/png'
  			}
  		}
  	}).catch(function (error) {
  		return q.reject('an error occured while sending a screenshot to the VisualReview server: ' + error);
  	});
  }
docs: Uploads an new screenshot to the given run.
  @param name
  @param runId
  @param metaData
  @param properties
  @param png
  @returns {Promise} a promise. If an error has occured, the promise will reject with a string containing an error message.