Support for Swift 4

[Jeehut]

Currently support for Swift 4 is not given due to the dependency CryptoSwift. It was solved already though in the develop branch and I'm sure a release is gonna be made soon. Until then I created #75 for all who can't wait to solve this issue.

[kylef]

https://github.com/kylef/JSONWebToken.swift/releases/tag/2.2.0 works with Swift 4.