associative fix for #156

#156 
What I did is:

* new methods to define the result as associative or not (`non_associative`, `associative`, `reset_associative`
* on the constructor I set the flag to the defined in config `self::$_config[$this->_connection_name]['find_many_primary_id_as_key']`
* on `find_many` check :
    -  if there is a primary key on the result instance  (we assume primary key is unique)
    -  if we look for an associative result set or not. (defined by the new config param and the new methods (`non_associative`, `associative`, `reset_associative`)


So, the config dictates the behaviour to return or not associative keys. That behaviour could be changed during fluent query. If there isn't a primary key define it fallbacks to non associative result set.