Adapted to build on OS X

Hi,
First, thank you so much for having made this nice and easy class, it's so helpful. I used it many times on iOS, it's so much time saving. As I'm using it now on a Cocoa (OS X) app, I fixed 2 issues I had:
- `kSecAttrAccessGroup` is unknown of OS X, so I disabled these lines when building on OS X
- using `OSStatus` with `%ld` in formatting strings creates a warning as `OSStatus` is just an int `not` a `long`
If you can pull it down, it would be great as I'm sure other Mac developers would love to use it as well.
Thanks!

-Micha