Add support for Asciidoc overview files.

- Only render files ending in .ad, .adoc, .asciidoc or .txt.
- Fallback to standard doclet for other file types.
- Add src/main/java/overview.adoc as example.
Fixed callout xml comments.
Strip leading indent from pom.xml snippet.
Add rendering of enum constant value doc comments.

(picked #24)
Fix #21, incorrect javadoc summary lines

- Use simpler section & paragraph output templates so javadoc can correctly
  extract the first sentence for its package, class & method summaries.
- Templates are bundled as resources in asciidoclet.jar then written to a
  tmp dir on javadoc execution where Asciidoctor can find them.
- Added Guava dependency to simplify copying of resources.

(picked #23)
Use Guava's File.toString() to read overview file.
Merge pull request #20 from opencloud/overview

Add support for Asciidoc overview files.
Add proper enum rendering.
fixed javadoc summaries.
Merge branch 'master' of github.com:asciidoctor/asciidoclet