Postgres driver.

This driver was originaly written by Alex Shevlakov using dbf driver
(PostgreSQL driver forked from DBF driver by Radim Blazek).
DBF driver was not good starting point, so I (R. Blazek) have rewritten it again
using ODBC, postgres driver and adding new code.

Driver supports only some field types, other types are ignored and warning is printed 
by G_warning().

FIELD TYPES:
Field type code may change. See for example
http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/catalog/pg_type.h

Because of this, type codes must be read from server when database is opened.

Supported types:
DB_C_TYPE_INT:
int2, smallint, int4, int, integer, int8, bigint, serial, oid

DB_C_TYPE_DOUBLE:
float4, real, float8, double precision, numeric, decimal

DB_C_TYPE_STRING:
character, char, character varying, varchar,  text, bool, boolean

DB_C_TYPE_DATETIME:
date, time, timestamp