Organized classes into Xcode folders.
Implemented NSCopying for MASShortcut.

This makes it possible to use shortcuts as collection keys.
Made the keyCode and modifierFlags properties read-only.

A shortcut is a good value type, it makes good sense to represent it
using an immutable object like NSNumber or NSString.
Implemented equality for shortcut objects.

Shortcuts are equal if and only if their key codes and modifier flags are.