Add default stylesheet and attribute support

This should fix #19, providing default stylesheets that render Asciidoctor output more nicely than the default.

- There are two new stylesheets, based on Java 8 and Java 6 javadoc stylesheets, with additions from Asciidoctor's default stylesheet. These also work fine on Java 7 and 1.5 javadoc output. The doclet detects the Java version and copies the appropriate stylesheet to the output directory, if no user stylesheet was specified. 

- Added the -attributes option, so that users can pass document attributes to Asciidoctor.

- Added some documentation to the README on the new options and also an Ant example.