Prevent blindly creating a JPEG.

This was part of the problem with creating JPEG2000 images.
Improve JPEG-2000 generation.

Now, we have the option of decompressing TIFFs automatically, to
allow the usage of the usual Kakadu, in addition to some checks if
ImageMagick supports JPEG-2000s.
Merge pull request #41 from adam-vessey/7.x

Improve generation of JP2s...