pgsql: Add GIN support for pg_trgm.

Log Message:
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Add GIN support for pg_trgm. From Guillaume Smet <guillaume.smet@gmail.com>
with minor editorization by me.

Hstore improvements
* add operation hstore ? text - excat equivalent of exist()
* remove undocumented behaviour of contains operation with NULL value
* now 'key'::text=>NULL returns '"key"=>NULL' instead of NULL
* Add GIN support for contains and exist operations
* Add GiST support for exist operatiion
* improve regression tests

Modified Files:
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    pgsql/contrib/hstore:
        Makefile (r1.4 -> r1.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/hstore/Makefile.diff?r1=1.4&r2=1.5)
        README.hstore (r1.4 -> r1.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/hstore/README.hstore.diff?r1=1.4&r2=1.5)
        hstore.h (r1.3 -> r1.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/hstore/hstore.h.diff?r1=1.3&r2=1.4)
        hstore.sql.in (r1.3 -> r1.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/hstore/hstore.sql.in.diff?r1=1.3&r2=1.4)
        hstore_gist.c (r1.3 -> r1.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/hstore/hstore_gist.c.diff?r1=1.3&r2=1.4)
        hstore_op.c (r1.5 -> r1.6)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/hstore/hstore_op.c.diff?r1=1.5&r2=1.6)
        uninstall_hstore.sql (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/hstore/uninstall_hstore.sql.diff?r1=1.2&r2=1.3)
    pgsql/contrib/hstore/data:
        hstore.data (r1.1 -> r1.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/hstore/data/hstore.data.diff?r1=1.1&r2=1.2)
    pgsql/contrib/hstore/expected:
        hstore.out (r1.3 -> r1.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/hstore/expected/hstore.out.diff?r1=1.3&r2=1.4)
    pgsql/contrib/hstore/sql:
        hstore.sql (r1.3 -> r1.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/hstore/sql/hstore.sql.diff?r1=1.3&r2=1.4)

Added Files:
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    pgsql/contrib/hstore:
        hstore_gin.c (r1.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/hstore/hstore_gin.c?rev=1.1&content-type=text/x-cvsweb-markup)