Put the literals into a separate namespace; resolves #31 (#32)

tests/sqlite/test_insert_by_ref_and_read.cpp

@@ -25,6 +25,7 @@ TEST(sqlite, test_insert_by_ref_and_read) {
            .id = 3, .first_name = "Maggie", .last_name = "Simpson", .age = 0}});
 
   using namespace sqlgen;
+  using namespace sqlgen::literals;
 
   const auto people2 = sqlite::connect()
                            .and_then(begin_transaction)