Only use pqxx::pqxx_exception with libpqxx 3.0 and later
Specify treatment explicitly for pqxx 2.6
Merge pull request #33 from tomhughes/exception

Only use pqxx::pqxx_exception with libpqxx 3.0 and later
Merge pull request #34 from tomhughes/treatment

Specify treatment explicitly for pqxx 2.6