Derivs: Replace "identify" with black/whitelisting.
Allow switchable derivative strategies.

Refactor default ImageMagick code into ExternalImageMagick strategy.
Fix basic tests for new derivative creator.
Remove useless file.
Add Imagick derivative strategy.

Refs #148.
Explicitly free Imagick resources.
Make "no thumbnails" notices account for other strategies.
Fix handling of images we can't make derivs for.
Add simple default option support.
Merge branch 'master' into derivatives
Merge branch 'derivatives'