Move Cucumber-related classes to a separate package

[php-coder]

Currently CucumberRunner and StepDefinitions classes reside in ru.mystamps.web.tests package. Let's create a dedicated package for Cucumber-related classes -- ru.mystamps.web.test.integration.cucumber. Also I suggest to put all step definitions to ru.mystamps.web.test.integration.cucumber.step package.

Tech debt for f72c87d (#46)

[olithin]

fixed in brunch -task644