Improved antialiasing of OpenCV thumbnails

Changes the OpenCV engine to resize using the INTER_AREA resampling method, which is the preferred resampling method according to http://opencv.willowgarage.com/documentation/python/imgproc_geometric_image_transformations.html#resize. 

This resampling method yields visibly better results than the default method (bilinear interpolation).

Before:
![bilinear](https://f.cloud.github.com/assets/103448/642504/80c3aeec-d35e-11e2-8a6c-9c7487867ddb.jpg)

After:
![interarea](https://f.cloud.github.com/assets/103448/642506/8b3baadc-d35e-11e2-9ae4-cd1ddc5f4e6c.jpg)

(Original image: http://cdn.cutestpaw.com/wp-content/uploads/2012/01/Hipster-llama-l.jpg)

Credit goes to @arthens for this fix.