Impl getRelPath method

The implemented getRelPath method is arguably the most complicated
CodeFile instance method.  The getter is a wrapper for a recursive call
in the form of 'recRelPath'.

'recRelPath' takes the absolute path of the file, and the cwd, and
recursively processes them.  The method views each path as a series of
'/' delimited tokens.  On each recursive call, a token is removed from
the paths, and the remaining paths are processed.

This method relies heavily on the use of the '/' character as a
delimiter.  Therefore, it is currently largely system dependent
(would break quickly on a Windows system).  Additionally, the
assumed use of the '/' delimiter may cause problems when files
contain a '/' character in their name.  Just making a note of this.