Add docstrings. Edit licence text.

Added docstrings to all the classes and some of the methods.
Corrected what I assume is a typo in the licence text,
change "may be used" to "may not be used". Removed some
commented-out test code in the main method.
Convert status and timestamp; fix Ditz YAML problems.

 - Map Ditz status string to nearest equivalent BE status.

 - Convert Ditz timestamp string format to BE format.

 - Preprocess Ditz YAML to fix a couple of deviations from
   the YAML specification which would otherwise confuse yaml.load.
Convert comments on Ditz issues.

Any comments on a Ditz issue are now converted and included in
the XML output.
Map Ditz releases to BE targets.

Ditz release names are now read from project and issue files,
and recreated as BE targets in the XML output.
Add readme and conversion script.

convert.sh is a simple script to automate the process of
exporting ditz data to XML and importing the XML to a BE
database.
Minor code fixes and tidy-ups.

 - Split one-line if statement in make_comment

 - Remove redundant assignment of issue_io in main

 - Correct a couple of minor typos in comments and docstrings