python - pq: Use cStringIO if available.

Log Message:
-----------
Use cStringIO if available.
Remove references to the strio object in the Passive buffer tests.
Add a simple test that pushes a fairly large message on the buffer.

Modified Files:
--------------
    pq/src:
        buffer.py (r1.2 -> r1.3)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/src/buffer.py.diff?r1=1.2&r2=1.3)
    pq/test:
        buffer.py (r1.2 -> r1.3)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/pq/test/buffer.py.diff?r1=1.2&r2=1.3)