objc.m Rewritten.

The original objc.m used NSLog() which:
1) generated a lot of useless output, and
2) did not actually used Objective-C objects.

I wrote a slightly complicated one using NSFileHandle, which:
1) write a short line to stderr without any extra data,
2) demonstrated using of Objective-C objects, and
3) is complete object-oriented code.

In this example, I used 3 classes, NSString, NSData and NSFileHandle.

Also, this example included a build instruction for both OS X using Xcode, and Linux using clang and GNUstep. Code asks for Objective-C ARC.