Added @Extra to pass extra options to dio requests, response, transformer and interceptors.

Example :

@http.POST('/path/')
@dio.Extra({'my_key':'my_value'})
Future<String>> myMethod();

Fixed general dart style and code conventions

Automatically null check with ArgumentError.checkNotNull for required parameters

Now SuperClasses can use forwarding/redirecting constructors instead of static instance() method

Example :

@RestApi(baseUrl: "https://httpbin.org/")
abstract class RestClient {
    /// Forwarding constructor
    factory RestClient([Dio dio]) = _RestClient;
}