Thanks to newenigma for catching.

Prefer the postMessage version to retrieve the metadata so that we can leave the existing JSON format as-is for backward compatibility with pre-1.424 Jenkins.

Unclear whether 'function() {...}();' is syntactically valid (works in Chrome & HtmlUnit tests) or whether we need '(function() {...})();' as Rhino seems to demand and various other scripts use.

Continuing c9cf3b67, retain jenkinsRule but empty as a placeholder, and register its rules in the new way.
Registering them with increasing positions, since there seems to be some implicit order dependency involving TR.rowvg-start.

Not so visible an effect currently due to a bug fixed in 4ace69e (maven-hpi-plugin 1.87)
by which extensions of all plugins which are test deps of jenkins-test-harness were being loaded inadvertently.
Originally tried to define ${bundledPlugins} using gmaven-plugin as per [1] but this does not work,
so adopted the tack of just using a static glob *.hpi and interpreting that in core.
[1] http://stackoverflow.com/questions/7601835/why-cant-the-maven-deploy-plugin-resolve-my-custom-system-property

Retain hudsonRules/jenkinsRules for now, as many scripts in core & plugins directly modify this hash.