Remove all using directives for STL namespace and members

Made all STL usages explicit to be able automatically find all usages of
particular class or function.

samples/cpp/matching_to_many_images.cpp

@@ -57,7 +57,7 @@ static void readTrainFilenames( const string& filename, string& dirName, vector<
 
     size_t pos = filename.rfind('\\');
     char dlmtr = '\\';
-    if (pos == String::npos)
+    if (pos == string::npos)
     {
         pos = filename.rfind('/');
         dlmtr = '/';