Replace include guards with "#pragma once" in all header files and in the database model generator scripts. (#506)

include/sqlpp11/insert.h

@@ -1,3 +1,5 @@
+#pragma once
+
 /*
  * Copyright (c) 2013-2015, Roland Bock
  * All rights reserved.
@@ -24,9 +26,6 @@
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#ifndef SQLPP11_INSERT_H
-#define SQLPP11_INSERT_H
-
 #include <sqlpp11/statement.h>
 #include <sqlpp11/connection.h>
 #include <sqlpp11/type_traits.h>
@@ -125,5 +124,3 @@ namespace sqlpp
     return {blank_insert_t<Database>().into(table)};
   }
 }  // namespace sqlpp
-
-#endif