Add Xcode project

I thought that since we have a Visual Studio project we could have a Xcode project as well. The project contains an extra target, dummy, to support code complete and similar features. Apparently Xcode doesn't support those features for external build systems, aka make. Any new files that are added should be added to the dummy target. One also needs to explicitly set the language for new files since DMD uses .c file extension instead of .cpp.