resolves #200 upgrade to Asciidoctor 0.1.2

- upgrade asciidoctor gem to 0.1.2
- add .ad as an extension for AsciiDoc files
- set the notitle! attribute so document title is displayed
- set section idprefix & idseparator to be consistent w/ other renderers
- set host-github to allow content to be shown when rendered on github
- fix test
- update install instructions for AsciiDoc