AsciiDoc content & config improvements

- upgrade Asciidoctor Gradle plugin to 1.5.2
- upgrade Gradle to 2.2.1
- fix header in index.asc
- use project version in header
- switch README to AsciiDoc
- fix broken callout number
Merge pull request #7 from mojavelinux/asciidoc-config

AsciiDoc content & config improvements