add 'can_comment_on_post' hook so we can better deal with the complications of Diaspora policy

doc/hook/can_comment_on_post.bb

@@ -0,0 +1,13 @@
+[h3]can_comment_on_post[/h3]
+
+Called when deciding whether or not to display a comment box for a post.
+
+
+Hook data (array):
+	observer_hash => xchan_hash of current observer
+	item => posted item
+	allowed => 'unset'
+
+
+To over-ride the default behaviour, change allowed to true or false
+