Adds sentinels support

[xnagpa]

Dear Faye::Redis developers
Please consider this request. The main idea of this pull requiest is to add Sentinel technology support. (Provides high availability for redis http://redis.io/topics/sentinel)
In order to achieve the desired result I used em-hiredis-sentinel gem. With this pull request applied one could use Sentinel technology by simply specifying the list of Sentinels when connecting to Faye.
Like

Faye::RackAdapter.new(
  :mount   => '/',
  :timeout => 25,
  :engine  => {
    :type  => Faye::Redis,
    sentinels: ['your_redis.com:26379', 'your_redis.com:26380', 'your_redis.com:26381'],
    master_name: 'mymaster',
  }
)

Sincerely yours Maksim