Added a keepAlive setting

- for config command restricted servers (etc. elasticache)
http://docs.aws.amazon.com/AmazonElastiCache/latest/UserGuide/ClientConfig.html

BookSleeve/RedisConnection
https://code.google.com/p/booksleeve/source/browse/BookSleeve/RedisConnection.cs

```csharp
        /// <summary>
        /// Configures an automatic keep-alive PING at a pre-determined interval; this is especially
        /// useful if CONFIG GET is not available.
        /// </summary>
        public void SetKeepAlive(int seconds)
        {
            keepAliveSeconds = seconds;
            StopKeepAlive();
            if (seconds > 0)
            {
                Trace("keep-alive", "set to {0} seconds", seconds);
                timer = new System.Timers.Timer(seconds * 500); // check twice in the interval; Tick will decide which (if either) to use
                timer.Elapsed += (tick ?? (tick = Tick));
                timer.Start();
            }
        }
```

If keepAlive not set, call to config command.
but Elasticache is restricted.

```csharp
        /// <summary>
        /// Called during connection init, but after the AUTH is sent (if needed)
        /// </summary>
        protected override bool OnInitConnection()
        {
            var result = base.OnInitConnection();

            if (keepAliveSeconds < 0) // not known
            {
                var options = GetConfigImpl("timeout", true);
                options.ContinueWith(x =>
                {
```


redis setting for test (redis.conf)
```
rename-command CONFIG b840fc02d524045429941cc15f59e41cb7be6c52
 or
rename-command CONFIG ""
```