Figured out how to serialize a shared_ptr<string>

Figured out how to serialize a shared_ptr<string>
My previous efforts at serializing a std::string stored as a shared_ptr
appear to have been foiled because the default shared_ptr serializaiton code
was expecting the class serializaiton code to be part of the class member
and alas std::string has out of class serialization.

So the simple solution was to make a subclass of std::string whose only
contribution was to define the serialization code. Also it turns out
there were some problems with the BOOST_SERIALIZATION_BASE_NVP macro
for the seq_string class, however when I expanded it manually, everything
seemed to work.