PL/Python: Convert numeric to Decimal

 PL/Python: Convert numeric to Decimal
 
 The old implementation converted PostgreSQL numeric to Python float,
 which was always considered a shortcoming.  Now numeric is converted to
 the Python Decimal object.  Either the external cdecimal module or the
 standard library decimal module are supported.
 
 From: Szymon Guz <mabewlun@gmail.com>
 From: Ronan Dunklau <rdunklau@gmail.com>
 Reviewed-by: Steve Singer <steve@ssinger.info>