Default to HTTP method GET for Alamofire.request()

This allows calling `Alamofire.request(URL)`. GET seems to be the most common request mode, so it makes sense to use it as a default. 

Implemented as a separate func because making `method` optional and unnamed would require re-ordering the params to avoid breaking existing implementations. I agree with having the request mode specified first anyway.

Also, added a test for it and updated the README.

Thanks for this great class!