ledger would abort with the following error message:
TypeError: No Python class registered for C++ class boost::optional<std::string>
The changes pass a CallPolicy to make_getter when adding the transaction
code property for python, so that the correct to_python conversion is
made. For details see:
http://www.boost.org/doc/libs/1_52_0/libs/python/doc/v2/faq.html#topythonconversionfailed
