Convert Image 5.6

Convert Image v1.6 changes

1) Better error notification if ImageMagick is not properly installed.

Fixes issue with those that have a thumbnail size set to something other than the XenForo default of 100.

In version prior to v1.4, if an image exceeded a maximum dimension in only one direction, the resize code only resized the image in one direction.

In v1.4 images will never exceed the allowed maximum width and height settings.

Fixed code so that large images now resize to max dimensions.

Fixed bug where converted attachments were shown as a link.

Fixed bug relating to animated gif images.