If running iOS >= 10 or macOS >= 10.12, use built-in system logging

[abbeycode]

The documentation is here: https://developer.apple.com/reference/os/1891852-logging

Switch logging to use a new macro (URKLog), with an ifdef that uses the existing NSLog behavior on older OS versions. All logging should happen on a custom logger whose identifier is published on the readme. The macro should take the log level as the first argument, and insert it as a string into the NSLog fallback.