removing breaks existing clients

I'd say: let's keep it, and sate in the doc that this method is deprecated.

The implementation would do a delegate like this:
```
-(void) login:(NSString*) username
     password:(NSString*) password
      success:(void (^)(id object))success
      failure:(void (^)(NSError *error))failure {
    [self login:@{@"username": username, @"password": password} success:success failure:failure];
}
```