pgsql: Significantly improve ranking: 1) rank_cd now use weight of

Log Message:
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Significantly improve ranking:
1) rank_cd now use weight of lexemes
2) rank_cd and rank can use any combination of normalization methods:
        no normalization
        normalization by log(length of document)
        -----/------- by length of document
        -----/------- by number of unique word in document
        -----/------- by log(number of unique word in document)
        -----/------- by number of covers (only rank_cd)

Improve cover's search.

TODO: changes in documentation

Modified Files:
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    pgsql/contrib/tsearch2:
        rank.c (r1.15 -> r1.16)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/tsearch2/rank.c.diff?r1=1.15&r2=1.16)
        tsearch.sql.in (r1.16 -> r1.17)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/tsearch2/tsearch.sql.in.diff?r1=1.16&r2=1.17)
    pgsql/contrib/tsearch2/expected:
        tsearch2.out (r1.18 -> r1.19)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/tsearch2/expected/tsearch2.out.diff?r1=1.18&r2=1.19)