updated pgMDBconstruct to fulfil PQconnectdbs specifications

PQconnectdbs parser doesn't expect empty values after a keyword. Therefore it doesn't work right, if a configuration string, e.g. the password is empty. Now strings are just added, if they contain sth.
Merge pull request #3 from matthiashh/hydro-devel

Updated pgMDBconstruct to fulfil PQconnectdbs specifications