import std::tr1/boost ::unordered_map into the factor namespace instead of using #define

vm/master.hpp

@@ -28,10 +28,16 @@
 
 #if __GNUC__ == 4
         #include <tr1/unordered_map>
-        #define unordered_map std::tr1::unordered_map
+
+namespace factor {
+    using std::tr1::unordered_map;
+}
 #elif __GNUC__ == 3
         #include <boost/unordered_map.hpp>
-        #define unordered_map boost::unordered_map
+
+namespace factor {
+    using boost::unordered_map;
+}
 #else
         #error Factor requires GCC 3.x or later
 #endif