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.

Now that the `swift-2.0` branch has been merged and deleted the migration guide is located on the `master` branch.

In README.md section "Creating a Manager with Background Configuration": replaced reference to deprecated method backgroundSessionConfiguration of NSURLSessionConfiguration with the new method as of iOS 8.0 called backgroundSessionConfigurationWithIdentifier