Add CocoaPods support

Like I said on twitter ^^ I think that's a really useful control and I'd like to see it on CocoaPods.

I take the liberty to rearrange the layout of the project to make it simpler to integrate the pod spec.

And the SDKit is not accesible as a pod, so I kept it as a submodule — like you did — and I temporary make it part of the `source_files` in the `SDLoginKit.podspec` file.