Add support for GROUP [N] BY syntax

[leogr]

Actually there's a typo in doc, so we missing the [N] param in GROUP BY:
http://sphinxsearch.com/bugs/view.php?id=1854

The [N] do this:
Starting with 2.2.1-beta, you can query Sphinx to return (no more than) N top matches for each group accordingly to WITHIN GROUP ORDER BY.