Merge pull request #6 from koher/dev-3.1.1

Develop 3.1.1

Merge pull request #5 from koher/dev-3.1

Support Swift 4.1

Merge pull request #4 from koher/dev-3.0

Develop 3.0.0

Reorder the properties of `Promise`

Update `description` for unfulfilled `Promise`

Fix Package.swift

Update README

Merge pull request #2 from koher/swift-3

Update for Swift 3

Reinitialize the package by SwiftPM

Implement `>>-?` and `-<<?`