BLD Release version 0.5

Fixing webp and improving installation (by automatically detecting it
and gracefully degrading if not found) are reasons enough to release a
new version. Because of the magic-number detection, it is better to
increase the version number.

Full ChangeLog:
    * Add magic-number based format auto-detection
    * Auto detect whether webp is installed
    * Fix WebP reading (update to newer API)