Support Swift package manager

[nicksnyder]

Documentation: https://swift.org/package-manager/

• [ ] Create Package.swift

import PackageDescription

let package = Package(
  name: "LayoutKit"
)

• [ ] Move Library code to a Sources/ folder?
• [ ] Anything else?
• [ ] Test Unit Tests
• [ ] Test Swift Package Manager by a sample project
• [ ] Test Cocoapods
• [ ] Test Carthage
• [ ] Update Readme.md

[jingwei-huang1]

Link to #118