Add Xcode static library project file

I needed to use your project but wanted to include it into an Xcode workspace to work with a project, so I needed to compile it as a static library. This pull request adds a project file that compiles `Reachability.m` and `Reachability.h` into a 'Reachability' static library.

To use it, add the project to a workspace and add the library to the project. `Reachability.h` can then be included in any project file as `#include "Reachability/Reachability.h"`.

Thanks for the code!