[PATCH] Fix conversion for Decimal arguments in plpython functions

Hi,
I've got a patch.

This is for a plpython enhancement.

There is an item at the TODO list
http://wiki.postgresql.org/wiki/Todo#Server-Side_Languages
"Fix loss of information during conversion of numeric type to Python float"

This patch uses a decimal.Decimal type from Python standard library for the
plpthon function numeric argument instead of float.

Patch contains changes in code, documentation and tests.

Most probably there is something wrong, as this is my first Postgres patch
:)

thanks,
Szymon