Add support for distribution via Swift Package Manager (SwiftPM)

[eneko]

This PR adds support for distributing XLForm via Swift Package Manager. This would allow users of Xcode 11 and above to add XLForm as a dependency using Xcode's integrated UI.

Summary of Changes

• Add Package.swift
• Flatten file structure to ensure consuming applications can find all public header files, without having to manually specify header search paths. Project structure in XLForm.xcodeproj has been unaltered.

Testing

To manually test distribution via XLForm:

1. On a new or existing project in Xcode
2. Go toFile -> Swift Packages -> Add Package Dependency...
3. Enter url https://github.com/eneko/XLFom
4. Choose branch master
5. Use XLForm in your app 🎉

[eneko]

@mats-claassen could you please re-run the CI build on Travis? CocoaPods was timing out this morning, but it's working again. Build is passing on my end: https://travis-ci.com/github/eneko/XLForm/builds/185000478

[mats-claassen]

Hi @eneko do you mind also adding a note to the Readme on how to install using SPM?

[eneko]

Hi @mats-claassen, sounds good. PR updated.

[eneko]

Thank you, @mats-claassen 🎉

When you have a chance, could you tag a new release? Both 4.1.1 or 4.2.0 would work.