feat(runner): Verfiy config.seleniumAddress or set null.

This PR introduces a feature that simplifies protractor configuration. If a seleniumAddress is provided in the config file, protractor smartly verifies this address is valid, else protractor sets seleniumAddress to null and moves on. With this feature, users can set both `seleniumAddress` and `seleniumServerJar` in the config. 

To verify the address we use node http.get(). Since this is async, we also wrap the main logic for `setUpSelenium` in a callback, `proceed`.