Implemented entity.GetType() in lua

This will only work for userdata decented from the Entity base class.
Using something else results in undefined behavior.

server/monsters/monster_data.cpp

@@ -21,6 +21,10 @@
 */
 #include "monster_data.hpp"
 
+MonsterData::MonsterData(): Entity("monster") {
+	//EMPTY
+}
+
 std::string MonsterData::SetAvatar(std::string s) {
 	return avatar = s;
 }