python - pq: Change state progression method.

Log Message:
-----------
Change state progression method. State is now progressed using the '>>'
operator, as opposed to directly setting the state. This creates a "transaction
protocol" that base Transactions need to implement. Really, it's not much better
than a named method, save it's apparent immutability(?).

Also, remove the error checking from client3.Transaction.__call__. This requires
that a Transaction be able to handle the error, or validate the type. This
allows transactions to actually handle errors, and to give more informative
tracebacks when a regular error occurs.

Make Function a bit more formal about its attributes and methods.

Modified Files:
--------------
    pq/src:
        client3.py (r1.10 -> r1.11)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/src/client3.py.diff?r1=1.10&r2=1.11)
        transam.py (r1.6 -> r1.7)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/src/transam.py.diff?r1=1.6&r2=1.7)
    pq/test:
        client3.py (r1.6 -> r1.7)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/test/client3.py.diff?r1=1.6&r2=1.7)