Set indexer name method

[adriaroca]

I create a new method to set the index name, specialy when the devoloper don't need the 'search' method, only 'query'.

In 'query' method, SphinxSearch use the first indexer specified in the configuration file. With this method, the developer can specify the indexer that is needed.

Example:

$sphinx = new SphinxSearch();
$sphinx->setMatchMode(\Sphinx\SphinxClient::SPH_MATCH_FULLSCAN);
$sphinx->setFilterFloatRange('@geodist', 0.0, $circle);
$sphinx->setSortMode(SphinxClient::SPH_SORT_ATTR_ASC, '@geodist');
$sphinx->setGeoAnchor('lat', 'lng', $lat, $lng);
$sphinx->limit(5000);
$sphinx->setIndexName('insurance_centers');
$results = $sphinx->query();