Start cleaning up the "prettythumb" vs "thumb" stuff in Ansel.

Start cleaning up the "prettythumb" vs "thumb" stuff in Ansel.

Now that styles have been refactored, we no longer need to differentiate
between a thumb and prettythumb thumbnail type, the style information
does that for us.

Simplifies the calls to all methods needing an image type as well as
allows us to get rid of the extra prettythumb.php image generation script.

Now, supported image types are "full", "screen", "thumb", and "mini". The style
object specifies what image generator to use, if supported.