Adding missing NSString methods from NSPathUtilities.h

These methods are part of Foundation (both on MacOS and iOS), but are declared at **[NSPathUtilities](https://gist.github.com/2974829)**. They were not available on maccore and I need them to do some file name cleanups (that can't be done on System.IO as System.IO doesn't handle symlinks).

This is a re-doing of https://github.com/mono/maccore/pull/30 from a clean branch.