Type error if no weather data returned

Love the module, great work.

I did find that if the OWM api returned no data (for example, it can't find your city - try an invalid string like '35dfgdsf234dfgd') an uncaught TypeError is thrown dying out of the process.

I have added in a check on the returned **cod** status code, to only start trying to step through the **.list** child object if data has been returned.

I'm a JS guy by day, so my coffeescript maybe a little off. Please feel free to come back with improvements.

Thanks