Copyright (c) 2012 Kevin Wellwood
All rights reserved.
This source code is distributed under the Modified BSD License. For terms and
conditions, see license.txt.
Calculates the signatures of all files in the 'inFiles' list and writes
a signed dictionary to 'filename'. If the dictionary file already
exists, it will be overwritten. An exception will be raised on error and
the partial dictionary file will be cleaned up.