Improve FileUtils#getImageJDirectory() method
Improve FileUtils#getImageJDirectory() method

It now works properly if you launch on the command line using Maven
("mvn exec:exec"). It also improves the documentation a bit.

Previously, commands like Help > About ImageJ... would not work in some
cases because it could not find the "about" folder with the images.

biovoxxel/imagej,biovoxxel/imagej,TehSAUCE/imagej,TehSAUCE/imagej,biovoxxel/imagej,TehSAUCE/imagej