python - be: Implement Transaction object.

Log Message:
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Implement Transaction object.

. Provide GT Transaction interface for the backend
. Add consistency checks when invoking code objects to validate that all opened
transactions are closed(aborted or committed).

Also,

. Move procedure execution outside of __main__. Use main to base the __plproc__
module on, but don't use it as it is not actually the usual __main__ context.
ie, in a procedure, __name__ == '__plproc__'.

Modified Files:
--------------
be:
setup.py (r1.27 -> r1.28)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/setup.py.diff?r1=1.27&r2=1.28)
be/src:
call.c (r1.15 -> r1.16)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/call.c.diff?r1=1.15&r2=1.16)
error.c (r1.12 -> r1.13)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/error.c.diff?r1=1.12&r2=1.13)
function.c (r1.25 -> r1.26)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/function.c.diff?r1=1.25&r2=1.26)
globals.c (r1.10 -> r1.11)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/globals.c.diff?r1=1.10&r2=1.11)
module.c (r1.32 -> r1.33)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/module.c.diff?r1=1.32&r2=1.33)
pl.c (r1.44 -> r1.45)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/pl.c.diff?r1=1.44&r2=1.45)
prime.py (r1.12 -> r1.13)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/prime.py.diff?r1=1.12&r2=1.13)
be/include/pypg:
externs.h (r1.11 -> r1.12)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/include/pypg/externs.h.diff?r1=1.11&r2=1.12)

Added Files:
-----------
be/src:
xact.c (r1.1)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/xact.c?rev=1.1&content-type=text/x-cvsweb-markup)
be/include/pypg:
xact.h (r1.1)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/include/pypg/xact.h?rev=1.1&content-type=text/x-cvsweb-markup)