pl/python quoting functions

Here's a patch that adds a few PL/Python functions for quoting strings.
It's an incremental patch on top of the plpython-refactor patch sent in
http://archives.postgresql.org/message-id/4D135170.3080705@wulczer.org.

Git branch for this patch:
https://github.com/wulczer/postgres/tree/functions

The new functions are plpy.quote_literal, plpy.quote_nullable and
plpy.quote_ident, and work just like their sql or plperl equivalents.

Cheers,
Jan