Adds carthage support

[Goos]

...by adding an xcode project.

[zacwest]

@Goos I thought the _Pods.xcodeproj symlink exposed the Example/ project file, which allows you to use the framework target that's in there. Is that not the case?

[Goos]

@zacwest afaik carthage assumes there to be an .xcodeproj with the same name as the library/repo name. I'm getting an

Reason: Project [...]/ZSWTappableLabel/ZSWTappableLabel.xcodeproj cannot be opened because it is missing its project.pbxproj file.

error when I'm building without the changes.

[cassianodialpad]

I'm getting the following error when trying to build 3.2 with carthage: Dependency "ZSWTappableLabel" has no shared framework schemes for any of the platforms: iOS