Improvements

• Codable protocol support #923 (Thanks @wleii)
• Xcode 9.3 support #951 (Thanks @wongzigii)
• Improve string test for boolean value #947 (Thanks @otaviolima)
• Update Travis CI to Xcode 9.3 image #954 (Thanks @wongzigii)

Docs

• README improvements #953, #952, #929 and #504 (Thanks @wongzigii, @tomduncalf and @bojanstef)