code: @Pure
  	public static File makeRelative(File filenameToMakeRelative, File rootPath) throws IOException {
  		return makeRelative(filenameToMakeRelative, rootPath, true);
  	}
docs: Make the given filename relative to the given root path.
  
  @param filenameToMakeRelative is the name to make relative.
  @param rootPath is the root path from which the relative path will be set.
  @return a relative filename.
  @throws IOException when is is impossible to retreive canonical paths.