Allow overriding the default navImages prefix url

By setting the default prefix url to /images/ (instead of hardcoding it), the code now matches the documentation, which states

<blockquote>
Finally the
image paths, by default assume there is a folder on the servers root path
called '/images', eg '/images/zoomin_rest.png'.  If you need to adjust
these paths, prefer setting the option.prefixUrl rather than overriding 
every image path directly through this setting.
</blockquote>