Initial draft for DefineSection \ RenderSection. This change is based on
changes to the Razor parser to enable Razor to generate HelperResults for
section instead of void delegates.
* Paths rooted by a leading slash (e.g. /foo) are correctly resolved by the
PhysicalFileSystem. This change is an experiment to determine if we can
get away with not having virtual paths in WebFx.
* Additionally removing types (MetadataVirtualPathViewFactory,
VirtualPathAttribute) that are currently unused.