Added NSProgress

• Added NSProgress support

You can track progress using

[mapper.progress addObserver:self
                  forKeyPath:NSStringFromSelector(@selector(fractionCompleted))
                     options:NSKeyValueObservingOptionNew
                     context:NULL];

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context {
    if ([keyPath isEqualToString:NSStringFromSelector(@selector(fractionCompleted))] && [object isKindOfClass:[NSProgress class]]) {
        NSLog(@"Mapping progress = %f", [change[@"new"] doubleValue]);
    } else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

It also supports cancellation

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
    [mapper mapFromRepresentation:JSON mapping:employeeMapping completion:^(NSArray *mappedObjects, NSError *error) {
        NSLog(@"Mapped objects %@ - Error %@", mappedObjects, error);
    }];
});

[mapper.progress cancel];

• Added a basic sample on view controller