Config-level connection for session (and cache?) #17

Hi,

I'd like to separate data in different databases, in particular sessions from data, say respectively into database 1 and 2.

I noticed you can instantiate any RedisEntity by specifying a connection, so in principle I can do that. However, I'd find more usable/less error prone to setup the default connection for data, and configure session to use another connection.

A proposal follows, please let me know if you'd like a pull request.
E.

• changes to ARedisSession.php

  public $connection = null;
  
  /**
   * Initializes the application component.
   * This method overrides the parent implementation by checking if redis is available.
   */
  public function init()
  {
      if ($this->connection !== null)
          $this->setConnection($this->connection);
      else
          $this->getConnection();
      parent::init();
  }
  
  
  - example of main/config.php
  
  ```php
  'redis' => array(
      'class' => 'ext.redis.ARedisConnection',
      'hostname' => 'localhost',
      'database' => 2,
      'port' => 6379,
  ),
  'redis-session' => array(
      'class' => 'ext.redis.ARedisConnection',
      'hostname' => 'localhost',
      'database' => 1,
      'port' => 6379,
  ),
  'session' => array(
      'class' => 'ext.redis.ARedisSession',
      'connection' => 'redis-session',
  ),