Allow custom setting of true/false choices

both on deserialization and on serialization.

Preserve existing behaviour by setting false_choices to ('false', '0'),
true_choices to an empty tuple, false_val to 'false' and true_val to 'true'.

The new behaviour lets the caller use translated strings both in
deserialization and in serialization, as in:

    bool=Boolean(false_choices=(_('no'),_('false'),_('n'),_('f'), '0'),
                 true_choices=(_('yes'),_('true'),_('y'),_('t'), '1'),
                 false_val=_('false'), true_val=_('true'))
Add myself to CONTRIBUTORS.
Allow specifying custom represnatations of values for boolean fields.