unify required, null, and drop

When I was trying to introspect schemas I printed out "missing:
<object object at 0x7fa8787da240>", "missing: <colander.null>" and
"missing: <colander._drop object at 0x7fa8737e4978>".  The first is
supposed to be a colander.required but unfortunately it's just a
generic object placeholder.  This PR is to unify colander.required,
colander.drop and colander.null.

colander/__init__.py

@@ -22,7 +22,12 @@
 
 _ = translationstring.TranslationStringFactory('colander')
 
-required = object()
+class _required(object):
+    """ Represents a required value in colander-related operations. """
+    def __repr__(self):
+        return '<colander.required>'
+
+required = _required()
 _marker = required # bw compat
 
 class _null(object):
@@ -46,7 +51,8 @@ class _drop(object):
     Represents a value that should be dropped if it is missing during
     deserialization.
     """
-    pass
+    def __repr__(self):
+        return '<colander.drop>'
 
 drop = _drop()