GSIP84

Hi,

this patch is an implementation of proposal GSIP84:
http://geoserver.org/display/GEOS/GSIP+84+-+Control+how+layer+groups+are+exposed+in+the+WMS+capabilities+document

I've added Type, Root Layer and Root Layer Style properties to Layer Groups.
I've updated  configuration UI, WMS GetCapabilities and GetMap services.