Make a function(ToTempSummon()) which can convert Creature to TempSummon with a C++ cast(reinterpret_cast) and with a type check(isSummon()).

This function is like ToPlayer(), ToTotem(), etc.

This commit is like ede831bdd6f0cff481acc33f269fa7f8c78befd4 / r7496:
http://code.google.com/p/trinitycore/source/detail?r=ede831bdd6f0cff481acc33f269fa7f8c78befd4
As a result, this commit has the advantages and the "disadvantages" too, like r7496.

Please try to be polite if this commit causes some crashes.

--HG--
branch : trunk

src/game/Vehicle.cpp

@@ -105,7 +105,7 @@ void Vehicle::Uninstall()
     for (SeatMap::iterator itr = m_Seats.begin(); itr != m_Seats.end(); ++itr)
         if(Unit *passenger = itr->second.passenger)
             if(passenger->HasUnitTypeMask(UNIT_MASK_ACCESSORY))
-                ((TempSummon*)passenger)->UnSummon();
+                passenger->ToTempSummon()->UnSummon();
     RemoveAllPassengers();
 }