OS Version related fixes:
- Fixes the Carbon.IsYosemite() and related functions by using the AppKit version number instead of Gestalt().
- Fixes the Carbon.SystemVersionAsString() function by using NSProcessInfo.OperatingSystemVersionString instead of Gestalt().
- Adds Cocoa.NSAppKitVersionNumber
Merge pull request #170 from tempelmann/master

OS Version related fixes:
- Fixes the Carbon.IsYosemite() and related functions by using the AppKit version number instead of Gestalt().
- Fixes the Carbon.SystemVersionAsString() function by using NSProcessInfo.OperatingSystemVersionString instead of Gestalt().
- Adds Cocoa.NSAppKitVersionNumber