Flatten the file hierarchy.

The division between detail and main wasn't making much sense.

[SVN r59317]

code_snippet.cpp

@@ -15,11 +15,11 @@
 #include <algorithm>
 #include <iterator>
 #include <boost/lexical_cast.hpp>
-#include "./detail/utils.hpp"
-#include "./detail/markups.hpp"
-#include "./detail/actions_class.hpp"
-#include "./grammars.hpp"
-#include "./code_snippet.hpp"
+#include "utils.hpp"
+#include "markups.hpp"
+#include "actions_class.hpp"
+#include "grammars.hpp"
+#include "code_snippet.hpp"
 
 namespace quickbook
 {