Convert example project to Carthage

[JoeMatt]

In relation to the partial add of Carthage in PR #12

Carthage support is partial since Carthage tries to use the XCode project to build the frameworks when included as a dependency in another project.
The current XCode project only builds after a executing pod install

Current workaround is to

carthage update --no-build
pushd Carthage/Checkouts/sReto/Source/
pod install
popd
carthage build

In order to fix this issue, the example project would need to be fixed to build successfully without executing any additional commands.
This could be done in 2 ways,

1. Include the sub-dependency projects directly in this repo after a pod setup by modifying the .gitignore rules to include the checked out source.
2. Change the project to use Carthage instead of cocoapods, since Carthage will find the sub-dependencies listed in this projects Cartfile, checkout and build and create proper symlinks to the shared checkout after resolving version mappings.

I suggest solution 2, which would still maintain Cocoapods support side by side to Carthage support.

[JoeMatt]

Feel free to assign this to me, added this ticket to track progress as noted in #12 .

[krusche]

Sounds like a good suggestion. As long as we still support CocoaPods for the main sReto framework, I am happy :-) Feel free to create a PR.

I would like to create another CocoaPod release in the next days as well. I will then wait until you have fixed this issue.

[JoeMatt]

I updated the title and description, I kept typing Cocoapods when I meant Carthage 🤦‍♂️

[krusche]

Alright :-) I think we should look to Alamofire for guidance as I did this in the past as well (a long time ago when Carthage was not so popular). They have CocoaPods and Carthage support and they also have an example project. We might also be able to support Swift packages in the future.