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>