Also cleaned up all OS version checks that are no longer necessary.

Add the `public` identifier to the `BackendError` enum in the part about using Generics for Serialization.

Reason:
The extension of Request throws an error ("Method cannot be declared public because its parameter uses an internal type") unless the enum is public.

Update cocoapod install to be compatible with cocoapod 1.x as it now requires a concrete target.

Search problem takes about five minutes, but it is much easier to see commands that just doing the right thing.

Xcode 7.1 is out in the App Store, the comment related to rdar://22307360 is no longer needed

I felt the description and instructions for *Carthage* weren't accurate.

- reworded the introduction to *Carthage* to be closer to how it describes itself.
- added project integration step instruction for *Carthage* based on its documentation.