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

This reverts commit f5ebe3b3. After more investigation, the `Response` struct initializer definitely needs to be public.

# Conflicts:
#	Alamofire.xcodeproj/project.pbxproj