model: *BUG* Fixing error regarding Freshness

Freshness policy was not behaving as expected.  Instead of keeping items
in the freshness container ordered by time, accidental use of
"push_back" method instead of "insert" completely invalidated the ordering.

Closes #41 (http://github.com/NDN-Routing/ndnSIM/issues/41)
Refs #1003 (http://redmine.named-data.net/issues/1003)
diff --git a/model/cs/custom-policies/freshness-policy.h b/model/cs/custom-policies/freshness-policy.h
index d22b624..5fa2a34 100644
--- a/model/cs/custom-policies/freshness-policy.h
+++ b/model/cs/custom-policies/freshness-policy.h
@@ -110,7 +110,7 @@
 
             // push item only if freshness is non zero. otherwise, this payload is not controlled by the policy
             // note that .size() on this policy would return only number of items with non-infinite freshness policy
-            policy_container::push_back (*item);
+            policy_container::insert (*item);
           }
 
         return true;