Update requirements to iOS 6 and OS X 10.8

It looks like the Xcode project specifies a minimum or OS X 10.8 SDK, and uses iOS 6.0 only features (strong properties with a type of `dispatch_queue_t`)
Minimum iOS version required for the Xcode project is 8.0
Merge pull request #1625 from tonyarnold/patch-1

Update requirements to iOS 8.0 and OS X 10.8
Code style fix (space -> tab)
Merge pull request #1637 from kkazuo/patch-1

Code style fix (space -> tab)
Update to Carthage 0.5 directory structure
Use a Cartfile.private, loosen version constraints
Merge pull request #1658 from ReactiveCocoa/carthage-0.5

Migrate to Carthage 0.5
Merge branch 'master' into swift-development

Conflicts:
	.gitmodules
	Cartfile
	Cartfile.resolved
	Carthage/Checkouts/Quick
	ReactiveCocoa.xcworkspace/contents.xcworkspacedata