python - fe: Add 'with' statement support to Transact.

Log Message:
-----------
Add 'with' statement support to Transact.

Also, return command completion messages from prepared statements. However,
parse and return the count when a sequence of digits are found in the string.

    from __future__ import with_statement

    C = Con()
    with C.Transaction:
        do_stuff_in_xact()

Modified Files:
--------------
    fe/src:
        tracenull.py (r1.23 -> r1.24)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/fe/src/tracenull.py.diff?r1=1.23&r2=1.24)
    fe/test:
        tracenull.py (r1.11 -> r1.12)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/fe/test/tracenull.py.diff?r1=1.11&r2=1.12)