Added support for Dark Messiah engine and game. (no bug, r=me).

The development of this feature would not be possible without the support of the following people from the game's community:
Dylan Riggs, Carl Pettengill, Ed Moreland, and Christian.

core/smn_halflife.cpp

@@ -445,6 +445,10 @@ static cell_t GuessSDKVersion(IPluginContext *pContext, const cell_t *params)
 	{
 		return 10;
 	}
+	else if (version == SOURCE_ENGINE_DARKMESSIAH)
+	{
+		return 15;
+	}
 	else if (version == SOURCE_ENGINE_EPISODEONE)
 	{
 		return 20;