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)