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.
Add -attributes-file option.

Allows attributes to be passed to the doclet using a file (a regular Asciidoc file).
Attributes will be passed to Asciidoctor when rendering javadoc comments.

Added hamcrest-library dependency for some tests, and also updated to junit-4.11 which
plays better with hamcrest.
refresh styles; update Asciidoctor & syntax to 1.5.0; tweaks
Removed test source block committed in error.
Update to depend on asciidoctorj-1.5.0.
Include usage from README rather than including directly pom.xml.

Workaround for asciidoctor/asciidoctor#1072, tagged regions not working in XML.
Update coderay and table styles from asciidoctor 1.5.0.
Updated to 1.5.0 monospace syntax.
Merge branch 'pr-26-revisions' of github.com:mojavelinux/asciidoclet into mojavelinux-pr-26-revisions

Conflicts:
	src/main/java/org/asciidoctor/Asciidoclet.java
Merge branch 'mojavelinux-pr-26-revisions' into stylesheets
Merge pull request #26 from opencloud/stylesheets

Add default stylesheet and attribute support