Miji.m: add jars in subdirectories

The Miji.m script adds Fiji.app/jars and Fiji.app/plugins to the MATLAB
classpath by explicitly iterating over *.jar in these directories. Thus
it was not recursing to subdirectories - notably /jars/bio-formats.

The script will now recurse through subdirectories to find jars.

Adapted from: http://stackoverflow.com/questions/2652630/
how-to-get-all-files-under-a-specific-directory-in-matlab
Miji.m convert strcat to fullfile calls

Instead of manually adding filesep let's let MATLAB decide how to make a
full file path.