DIP 43: D/Objective-C

Here it finally is: support for `extern(Objective-C)`. The matching pull request for druntime is here: https://github.com/D-Programming-Language/druntime/pull/957

http://wiki.dlang.org/DIP43
Please, no more boolean-named functions that return `int`. The ones we have at this point are confusing enough. Make it an enum if needed.