align URL format with libpq in documentation

[petere]

The documentation (README and wiki) uses random URL formats like tcp:// and pg://. Since node-postgres accepts anything as the scheme, it would be nice if the documentation aligned itself with libpq's format (http://www.postgresql.org/docs/9.2/static/libpq-connect.html#LIBPQ-CONNSTRING) using postgresql:// or postgres:// so that connection strings are more easily interchangeable. This would also improve the internal consistency of node-postgres's documentation.