Upgrade to Xcode 9.3 and Swift 4.1 (#66)

* Upgrade to Xcode 9.3 and Swift 4.1

* Bump .swift-version to 4.1

* Update Package.swift to swift tool-version 4.0

* Update changelog entry in readme for 2.2 release

bump version to 2.1

Merge branch 'xcode81' of git://github.com/madhavajay/Interstellar in…

…to madhavajay-xcode81

fix qos-deprecation warnings

add swift build to readme and package file

remove logging statements

remove threading from interstellar

add disclaimer about real frp in readme, fixes #2

add a readme