Improve antialiasing of OpenCV thumbnails

Resample using inter area interpolation. This is the preferred
resampling method according to [1], and yields visibly better results
than the default method (bilinear interpolation) for a minor impact on
performance.

[1]: http://opencv.willowgarage.com/documentation/python/imgproc_geometric_image_transformations.html#resize
Merge remote-tracking branch 'globocom/master' into improved-opencv-resampling
Merge pull request #189 from harto/improved-opencv-resampling

Improved antialiasing of OpenCV thumbnails