It has actually worked before (likely because it's inside a class),
but C++03 does disallow unnamed types as template arguments,
and there's a report that at least in one version of GCC it's
enforced: <http://code.opencv.org/issues/3278>.
Same prototype as BOWImgDescriptorExtractor, but do only the matching.
If the feature is accepted, the BOWImgDescriptorExtractor and BOWImgDescriptorMatcher should probably refactor with inheritance.
Add a class to compute the keypoints, descriptors and matching from an image should be added to.
It doesn't make sense to export template classes, since there's no way
the user can learn that it can import them. In fact, it is somewhat
deleterious, because every module will export every instantiation
that it uses, but doesn't inline.
