feat(runner): add adapter for cucumber.js

As a fix for #15 I make an adapter for [Cucumber.js](https://github.com/cucumber/cucumber-js) to Protractor.

Like the other adapters (Jasmine, Mocha), the Cucumber adapter is merged in lib/runner.js. (Not perfect, I think there is room for improvement here).

It adds a new configuration parameter `cucumberOpts` that can be used to pass parameter to the Cucumber instance. The `steps`, `tags` and `format` options are supported.

Config file example:
```javascript
exports.config = {
  seleniumAddress: 'http://localhost:4444/wd/hub',

  framework: 'cucumber',

  specs: [
    'cucumber/*.feature'
  ],

  capabilities: {
    'browserName': 'chrome'
  },

  baseUrl: 'http://localhost:8000',

  cucumberOpts: {
    require: 'cucumber/definitions',
    tags: '@dev',
    format: 'pretty'
  }
}
```
You can read the Cucumber.js CLI documentation for more informations about options.

I also added some tests (inspired by the Jasmine one's) in `spec`, they was added to the main `npm test` script.