Added Swift Package Manager support

[rwbutler]

This PR adds support for Swift Package Manager by adding a Package.swift file (I have tested this integration on my local branch where it integrates into a sample project without issue).

It also adds support for Carthage, I had thought that previous PR (https://github.com/gringoireDM/LNZCollectionLayouts/pull/28) would be enough to enable Carthage compatibility by simply marking the scheme as shared however as it turned out when I attempted to integrate LNZCollectionLayouts using Carthage there were a few more requirements.

In the end I had to add a header and an Info.plist for the framework as well (the existing Info.plist used by the sample application was renamed to AppInfo.plist with Info.plist used for the framework target and AppInfo.plist used the sample app) and resolved a couple of small compilation issues.

I currently integrate LNZCollectionLayouts via Carthage using my fork but hope that with this PR merged it should be possible to switch to using the official (this) repository.