pgsql: 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>

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/7919398bac8bacd75ec5d763ce8b15ffaaa3e071

Modified Files
--------------
doc/src/sgml/plpython.sgml                    |   23 +++++++++---
src/pl/plpython/expected/plpython_types.out   |   49 +++++++++++++++++++++----
src/pl/plpython/expected/plpython_types_3.out |   49 +++++++++++++++++++++----
src/pl/plpython/plpy_typeio.c                 |   42 ++++++++++++++++-----
src/pl/plpython/sql/plpython_types.sql        |    9 ++++-
5 files changed, 138 insertions(+), 34 deletions(-)