Make ASNetworkImageNode Just Work.

Use ASBasicImageDownloader by default -- you can now do...

  ASNetworkImageNode *imageNode = [[ASNetworkImageNode alloc] init];
  imageNode.URL = [NSURL URLWithString:@"..."];
  [self.view addSubview:imageNode.view];

...and it'll do the right thing.
Merge pull request #135 from facebook/basic-init

Make ASNetworkImageNode Just Work.