Added original SDK code for Alien Swarm.

game/server/bitstring.cpp

@@ -0,0 +1,27 @@
+//========= Copyright <20> 1996-2005, Valve Corporation, All rights reserved. ============//
+//
+// Purpose:		Arbitrary length bit string
+//				** NOTE: This class does NOT override the bitwise operators
+//						 as doing so would require overriding the operators
+//						 to allocate memory for the returned bitstring.  This method
+//						 would be prone to memory leaks as the calling party
+//						 would have to remember to delete the memory.  Funtions
+//						 are used instead to require the calling party to allocate
+//						 and destroy their own memory
+//
+// $Workfile:     $
+// $Date:         $
+// $NoKeywords: $
+//=============================================================================//
+
+#include "cbase.h"
+
+#include <limits.h>
+
+#include "bitstring.h"
+#include "utlbuffer.h"
+#include "tier0/dbg.h"
+
+// memdbgon must be the last include file in a .cpp file!!!
+#include "tier0/memdbgon.h"
+