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/scripts/northrend/utgarde_keep/utgarde_pinnacle/boss_svala.cpp

@@ -174,7 +174,7 @@ struct boss_svalaAI : public ScriptedAI
                     {
                         m_creature->SetFlag(UNIT_FIELD_FLAGS, UNIT_FLAG_NON_ATTACKABLE|UNIT_FLAG_NOT_SELECTABLE);
                         m_creature->SetDisplayId(DATA_SVALA_DISPLAY_ID);
-                        ((TempSummon*)pArthas)->UnSummon(); // TODO: should need a ToTempSummon()
+                        pArthas->ToTempSummon()->UnSummon();
                         ArthasGUID = 0;
                         Phase = FINISHED;
                     }