Some updates to the site.
The site now includes javadoc of all the default constituent parts of
Revapi (not linked from anywhere yet).

Added a simple new chapter on the architecture and integrated lightbox
into the site.
Javadocs, ChangeSeverity -> DifferenceSeverity, Java SPI in its own pkg.
Leave out site from the default build.
Left out some javadoc from the site only keeping the API portions.
Wrote some architecture docs for the site.
More javadoc.
Include the link to the javadocs in the site top menu.
Make the difference transform generic to specify the expected type of
the transformable elements.
Site updates.
Stab at site deploy.
Move Code to SPI, pull methods from AbstractJavaCheck up to CheckBase,
javadocs.
Added one more check for another flavor of introducing a new method.
Consolidated annotation type attribute removal checking.
Documented all the Java checks on the site and updated their severities
where I got it wrong the first time.