DescriptorExtractor: added defaultNorm() to return default norm type.

Added the defaultNorm() method to the DescriptorExtractor class. This method returns the default norm type for each descriptor type. The tests and C/C++ samples were updated to get the norm type directly from the DescriptorExtractor inherited classes.

This was reported in feature report #2182 (http://code.opencv.org/issues/2182). It will make it possible to get the norm type usually applied matching method for each descriptor, instead of passing it manually.

samples/cpp/freak_demo.cpp

@@ -96,7 +96,7 @@ int main( int argc, char** argv ) {
     // The standard Hamming distance can be used such as
     // BFMatcher matcher(NORM_HAMMING);
     // or the proposed cascade of hamming distance using SSSE3
-    BFMatcher matcher(NORM_HAMMING);
+    BFMatcher matcher(extractor.defaultNorm());
 
     // detect
     double t = (double)getTickCount();