Adding Mac OS X Carthage support.

[dani-mp]

I've:

• Created a new OSX target.
• Added files to the new target.
• Shared new scheme.
• Added an umbrella header for both targets.
• Tagged a new release to confirm that is working.

You would want to change the name of the iOS target/scheme to differentiate them better. Check out Alamofire, for example, that does this.

[naoty]

Fixed at https://github.com/naoty/Timepiece/pull/20.