Move main executable source to its own directory (#248)

* Move main executable files to their own directory

* Adapt the build to the new file structure

... and fix some weird #include's that were relying on the project root
being an include directory.

lua/lua_function.cpp

@@ -1,5 +1,5 @@
 #include "DEBUG.H"
-#include "lua/lua_function.h"
+#include "lua_function.h"
 
 template<>
 LuaFunction& LuaFunction::Param<std::string>(std::string const& par)

lua/lua_state.cpp

@@ -1,4 +1,4 @@
-#include "lua/lua_state.h"
+#include "lua_state.h"
 #include <Debug.h>
 #include "sgp_logger.h"
 

lua/lua_table.cpp

@@ -1,4 +1,4 @@
-#include "lua/lua_table.h"
+#include "lua_table.h"
 
 /*
  *  Table constructors

lua/lua_table.h

@@ -1,7 +1,7 @@
 #ifndef _LUA_TABLE_H_
 #define _LUA_TABLE_H_
 
-#include "lua/lua_state.h"
+#include "lua_state.h"
 
 class LuaTable
 {