Add possibility to run cucumber with BROWSER=iphone-simulation

[sjieg]

We can do this by adding more options in browser.rb. Beside forcing a certain browser size, we should also adjust the user agent. https://github.com/alisterscott/webdriver-user-agent

Possibly add a referance configuration file where u can add devices like:

• iphone
• • width
• • height
• • user agent string

[sjieg]

This issue can't be fixed until #66 is implemented.