wapi: add POST /uri/$DIRECTORY?t=set_children

Unfinished bits: doc in webapi.txt, test handling of badly formed JSON, return reasonable HTTP response, examination of the effect of this patch on code coverage -- but I'm committing it anyway because MikeB can use it and I'm being called to dinner...

src/allmydata/dirnode.py

@@ -250,7 +250,7 @@ class NewDirectoryNode:
         assert isinstance(name, unicode)
         return self.set_node(name, self._create_node(child_uri), metadata)
 
-    def set_uris(self, entries):
+    def set_children(self, entries):
         node_entries = []
         for e in entries:
             if len(e) == 2: