check for "keywords" in complexe "where" clauses

Allows to construct queries like :
Model::find('all', 'where' => array(
'key' => value,
array(
  array('key_2' => value_2),
  'or' => array('keywords', $keywords)
));
Merge pull request #1 from vrcAlbert/feature_recursive

Feature recursive search in complex "where" clauses