python - lo: Refactor the version parser.

Log Message:
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Refactor the version parser.

Relocate the parsing code into split() and make it so that split returns None in
cases where the piece of information is absent. This later proves useful for
general comparisons: "8" is "8.1" or "8.2" or "8.3". (pg_tin uses this for
version selectivity; pg_tin -V 8 com psql -c 'SELECT version()' # select 8.x)
(need to add less than and greater than methods still)

Modified Files:
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    lo/postgresql/utility:
        version.py (r1.2 -> r1.3)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/lo/postgresql/utility/version.py.diff?r1=1.2&r2=1.3)