In recent versions of libpqxx the pqxx::prepare::treat_string has been removed.

Please validate this code works as expected.
ed: support pqxx v3 and v4

As pqxx::prepare changed in pqxx v4 and that we need to support pqxx v3 because v4
is not yet in Debian stable, we check the pqxx version with the preprocessor.

fix #301.
Merge pull request #514 from TeXitoi/libpqxx4

ed: support pqxx v3 and v4