Initial node_middleware commit. Implements an attribute on the HttpRequest class...

Initial node_middleware commit. Implements an attribute on the HttpRequest class which handles lazy node evaluation - i.e. request.node is available for all requests, but is only evaluated on first use. Removed all references I could find to node, path, and subpath in Views, which may make for a simpler API. Path and subpath were generally unused and can be extrapolated in other ways; however, should it come to light that they are necessary, they can be added back in.