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 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.