make site reactor aware

there are a few things I'd like to do to make the sites more workable:

• it's good they can still build independantly
• however, if built from the parent, it would be good to go into aggregation mode and pull in those separately built sites, do any dashboards and deploy together
• some reports would aggregate themselves, as well as participate in the dashboards (eg javadoc)
• I'd like the navigation to behave hierachically (sections of the parent navigation marked as inherited remain on the child sites)
• this means that when building from independant projects they'd need to be able to locate the parent site descriptor. This could be tricky without a USD - should it fail if it is not a USD, or should the site descriptor be published/referenced externally somehow?