Add setters for replacing the default Edge and Node classes

This can be handy if the user wants to extend the functionality of those
classes. They can simply be inherited in a new class with that new class
being used in the Graph then.

fd/Graph.lua

@@ -242,4 +242,20 @@ function Graph.new()
     return self;
 end
 
+---
+-- Replaces the default Edge class with a custom one.
+-- @param class - The custom Edge class to use.
+--
+function Graph.setEdgeClass( class )
+    Edge = class;
+end
+
+---
+-- Replaces the default Node class with a custom one.
+-- @param class - The custom Node class to use.
+--
+function Graph.setNodeClass( class )
+    Node = class;
+end
+
 return Graph;