Feature - Added 204 Support to Response Serializers

Drop mention of rdar://22307360, Xcode 7.1 is out

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

replace unnecessary usage of NSString with String

Updated description and instructions for Carthage

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.

Update Alamofire 3.0 Migration Guide.md

Spelling fix