ZF2-219: Should be using associative arrays for joins in Zend\Db\Sql\Select.php

Description

On line 24 of Zend\Db\Sql\Select.php, the join is added with "$this->joins[] = array($name, $on, $columns, $type);", which then requires using $join[3] etc later on. I suggest we use associative arrays here for code clarity.

Comments

I could go with that.

Refactored in master.