resolves #200 upgrade to Asciidoctor 0.1.4

- upgrade asciidoctor gem to 0.1.4
- update install instructions for AsciiDoc
- configure Asciidoctor to output correct `<pre>` tags for code listings
- set the showtitle attribute so document title is displayed
- set section idprefix & idseparator to be consistent w/ other renderers
- set env-github to allow conditional content when rendering on GitHub
- add tests for admonition (no icon) and source listing
- set :safe to :secure just for piece of mind
- fix test