Allow solution pack to accept JP2 files natively

Also removes the module.install requirement that the kdu-compress executable be on the Apache PATH.  We're specifying the path as part of the configuration, so there is no reason why the executable should be available at module install time.

This module makes use of kdu-expand to create TIFF images from JP2 OBJ datastreams (e.g. where there was no original TIFF ingested but a TIFF derivative is needed), so the 'islandora_kakadu_url' variable was changed to point to the *directory* where the Kakadu executables live.  An update hook is provided to modify the variable.

Based on work by @grf (Randy Fischer of FLVC).