refactoring

- added missing Copyright note
- generated the ALL_BACKENDS map in a Groovier way
- reordered methods: public first then private (in usage order)
deferring creation of Asciidoctor instance to speedup task creation

- creating an Asciidoctor instance can take many seconds
- creating it in the constructor means that it's already
  run during task configuration
- so even if the asciidoctor task is not run it would take
  that long in Gradle's configuration phase
Merge pull request #22 from darxriggs/master

deferring creation of Asciidoctor instance to speedup task creation