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.
Updated to match latest asciidoctor version
Add DocletOptions to represent the doclet options we need.

Provides a clean way for other objects to access the doclet options.

Also refactored Asciidoclet a bit so it doesn't use a mutable static
variable for the standard doclet instance.
Fix #19, provide default stylesheets suitable for Asciidoctor output.

- Default stylesheet used when no "-stylesheet" option is given.
  Otherwise use the user's stylesheet.
- Detect Java version and copy appropriate stylesheet. Currently has
  stylesheets for Java 8 (& 7) and Java 6 (& 1.5) javadoc output.
- Stylesheets based on standard javadoc CSS with additional parts
  selected from Asciidoctor's default stylesheet.
Add support for setting document attributes.

- added -attribute parameter, takes list of key[=value] items.
- set javadoc attribute for use in ifdef/ifndef.
Add doclet options documentation.

- document new options in README.
- show examples of new options in various build systems.
- include usage sections of README in javadoc overview to avoid duplication.
Use coderay's CSS classes rather than inline styles.

The inline styles were setting some odd background colours that
made the output look ugly.
Removed unused author attribute, restored original author.
Merge branch 'stylesheets' into opencloud

Conflicts:
	README.asciidoc
	pom.xml
	src/main/java/org/asciidoctor/Asciidoclet.java
	src/main/java/org/asciidoctor/asciidoclet/AsciidoctorRenderer.java
	src/main/java/org/asciidoctor/asciidoclet/DocletOptions.java
	src/test/java/org/asciidoctor/AsciidocletTest.java
	src/test/java/org/asciidoctor/asciidoclet/DocletOptionsTest.java